When I see commercial hobbyist skimmers like aquaC, euroreef, PM, etc, I'm always amazed at how much they cost for what they are. (and those are the good ones!!) So I started making my own. I use top quality materials, build skimmers that work well enough so that many of the fish stores in my city use them for their reef systems, despite the fact that they get the commercial stuff at cost or lower, and I have had fun doing it in the process. If you need to buy tools like a router, table, bits, and you want to use high quality acrylic (I use 3/8" cell cast and 1/4" wall cell cast tube) it's probably not cheaper to build one; but you might enjoy it, maybe build a couple of extra ones for hobbyists in your area, and recoup some of your cost.
If you're skimming a 75 gallon, the easy way to go would be to build a recirculating skimmer running off the sedra 5000; (or an equivilent ocean runner) the pump does all the work, you just basically attach it to a tube, build a collection cup, some plumbing, off you go. (I'm oversimplifying somewhat!) A small beckett skimmer, say 15" or so of 6" diameter tube, driven by a 750-1000gph pressure pump, would be a much more powerful skimmer, but a little more complicated to build.