It's funny you should mention this. I just finished building this filter pictured below on Tuesday night. The only thing I have left is to water test it (which it should pass), drill two bulkheads, and put the rest of the bio-balls in. It will be running a 100 gallon FO system.
Now, why it's funny is that I looked into a few things while I was waiting for a joint to set. By the time I finished buying, I had purchased:
(2) 2x4 foot sheets of .220 Acrylic $70 total
(2) 1.5" bulkheads $25 total
(5) gallons bio-balls $45 total
I already had the egg-crate, and I don't count the purchase of the table saw blade (50) and Weld-On (20 or so) as I can use those for other projects.
So that adds up to $140. Looking online at a few places, CPR Aquatic (whom I am a fan as I already own 2 of their filters and CSOs) filters with the same bio-media capacity cost almost most the same (167 at a "major online aquatic retailer").
I think I still did pretty good as I saved 27.00, and I have a huge sump with fuge section to go with it (the CPR1000 has a tiny sump), plus it's exactly the way I wanted it (fit the stand, was modular, etc). But, if your sole purpose is to save money, buy commerically. It looks like that stuff is overpriced, but it's really not.
If you want advice on how to joint Acrylic, work with it, etc, I'd suggest looking online. Nobody here is going to be able to provide you with an illustrated, comprehensive overview without referring you to a site anyway.
(Edit: Forgot the pic)