If you go with a straight square tube design you won’t have any tricky angle edge cuts, much better if you’re not experienced with cutting mitered lumber, etc.
Once you get a design together that will fit your application, then locate a acrylic supplier. Hopefully they can cut it for you, otherwise a circular saw will work. If you need to cut the sheet, and you will if you go with ANY angled design such as mine (since the edges have to be angle cut where the top & bottom rest) you’ll have to get a hollow-ground blade for cutting laminates and clamp a straight edge to your sheet for accurate edges. YOU MUST KEEP BLADE COOL WITH A WATER SPRAY BOTTLE when cutting, this is a good time for a helper. Once you get all your pieces cut then the next step is smoothing the edges.
The router bit is a great tool for smoothing the edges, even if the shop cut your plastic, you’ll still need to smooth the saw marks from the edge. Just run the router bit down the edge, 
scraping as you go, don’t gouge into the sheet, just scrape. You’ll see the marks eventually disappear. Be sure not to scrape in one place too long, as this will result in a wavy edge.
Once the edges are smooth do a trial fitting, if there are no major gaps you’re ready to seam. Seam the four sides first, one at a time. Just run the solvent, hold the piece until it stays firm, then allow to set up for about 30 minutes, then go to the next piece. Once the sides are all seamed, make sure the bottom and top resting edges are all even and smooth, if not, get a large sanding block and some 200 grit wet-dri paper and wetsand the top and bottom until they’re smooth and square. Seam the bottom piece. Now is the time to prepare the top piece.
Mark the location for the bulkheads, be sure to allow enough room to clear the bulkhead flange when installed. Drill the proper size hole with your holesaw, but drill as follows: Drill only 1/8” into sheet, flip sheet over and drill rest of the hole, this will prevent splintering. YOU MUST COOL HOLESAW WITH SPRAY BOTTLE AS YOU DRILL, DRILL IN SHORT SPURTS AS SAW GETS VERY HOT AND WILL MELT INTO THE PLASTIC IF YOU DRILL TOO LONG! Trust me, I’ve learned the hard way. Once you get the holes drilled go ahead and LOOSELY install the bulkheads with the flange and rubber 
washer INSIDE the lid and the holding nut OUTSIDE the lid. Then thread your ½” or ¾” MPT-PVC slip adapter into the incoming bulkhead with a couple wraps of teflon tape, and tighten ‘til snug. Now measure your ½ or ¾” PVC down pipe (if your design is similar to mine and you need some elbows to center the pipe, be sure to allow for this now). Allow for any length of pipe that will be inside any fittings and extend the down pipe to come to ½” from the bottom of the filter if using ¾” PVC, and 3/8” from the botttom if using ½” PVC. This is important, too much of a gap and you get poor flow or “channeling” in the sand bed, too little of a gap and you restrict your pump output. Once you get your measurements, cut and glue pipe with PVC cement. Now be sure to tighten down your bulkheads, using a large pipe wrench or welding clamp helps, you want these bulkheads pretty darn tight!
Once you’ve got everything tight and the pipe is the proper length you can go ahead and seal the top.
Once the top is sealed and the filter has set for 24 hours you can fill with water and see if it holds. If you have good seams you’ll be fine!
Now it’s time to fill with gravel & sand. I use 1 part gravel to 5 parts Silica sand, the gravel allows you to maintain higher flow in the filter with less chance of blowing out sand, it also keeps the sand from clumping together. Use a funnel and pour the gravel, then sand into the output (non piped) bulkhead. You want to only fill your FBF so it’s about 1/3rd full of media. Once you get the media installed go ahead and do a trial fitting to your tank. You’ll need various “L”s and connectors as per your application. Here’s my setup to give you an idea.
As far as sizing your filter. I’d recommend going with ¾” PVC / fittings / bulkheads / valves, and a MagDrive 7 pump for tanks over 100 gallons. I’d also recommend between 7-15 pounds of media depending on what it takes to get 1/3rd volume level.
I’d recommend ½” PVC / fittings / bulkheads / valves, and a MagDrive 5 for tanks under 100 gallons and between 3-7 pounds of media.
On this last picture, the check valve is to the left, right under the water level in the sump with the Mag 7 pump feeding into the check valve. 
Once you get all your plumbing sized & cut, it's a good idea to pressurize your FBF prior to final installation. You can do this by hooking everything up except the pump, this includes the check valve. Put a garden hose on the incoming line and turn the hose on full pressure, this will clear any clogged media in the tube and allow easy startup w/ your pump, the checkvalve will keep the media from clogging the line once you remove the garden hose. Then connect the pump and it should flow just fine. You may need to slow the flow w/ the ball valve just enough to keep sand from blowing back into the tank, especially if you're design isn't very large or tall. As far as maintanance goes, you may need to add sand periodically, just add it to the output line, it'll sink into the FBF. Every 6-12 months it's a good idea to connect the garden hose up to the input of your pump and give it a good "flush" this will release some of the sticky sludge that can clump the sand together. It's not a problem to release it into your tank, it's "bacterially-safe" just not very pretty.
