i wat to point out one thing. Like others have said this is a low cost BOM for the field. I believe personally that on the official FIRST field, some of that plywood will be another material. I remember last year on the "cheap" BOM for the field, tht stated you could use plywood in replace for the lexan and HDPE.
~Mike
[edit]look at the image of 5" wheel casters i found, and they are from skyway (a FIRST supplier from past years)!
The large Black one is the 5" caster, i cant find a price though to verify that this could be the "casters" for $22 each.