The gearbox is designed to fit their colson hubs. Colsons come in a lot of sizes. Here is a 5" one that as far as I know will fit their hubs:
http://www.robotmarketplace.com/products/BPDWC14.html
Also if you want to use wider wheels what we did was tap the end of the shaft and put a spacer(we put hex, can be round) to get the extra length needed.
Keep in mind that the WCP product is an end all solution. It just works. If you design your own you will need to prototype(you will not get it right otherwise). Also you will have to make your own replacements if parts fail(WCP takes care of you on this). Lastly the time can just be better used elsewhere. Sure people may not like designing, but prototyping is very important. If you don't want to buy premade bearing blocks(WCP sells COTS ones too) than those need to be amchined for a WCD. Drive rails and shafts are more things that can be machined week 1.