Unless you can get donated frames or machining, I think the cheapest way to make a drive base is to buy a 2016 KOP from Andymark (
http://www.andymark.com/AM14U3-p/am-14u3.htm), especially when they come on sale infrequently, and US Digital encoders for the toughboxes (
http://www.andymark.com/E4T-OEM-Mini...-p/am-3132.htm)