Exactly, the PLYWOOD 3/4" sheet cut to size whether 4'x6' or 4'x5" bolted to a 2" x 4" frame will provide stress bracing in whatever direction is needed and provide a "floor" for the body which could have some large holes cut for underneath spraying. Four 12" diameter holes in the corners would leave cross strength in the middle as well as room for some spraying underneath as long as the caster mounts get it up off the floor. Although there are extra long deck screws, probably 1/4" x 6" thru bolts are more reliable. I went through this design-think to support a 1500 pound magnet on a caster wheel roller and used a 1" thick aluminum plate on top with steel perimeter and had to add vertical braces at each corner to get the deck 14" up off the floor. As said above you need to get the rating on the casters and diameter of the wheels large enough for easy rolling. This reminds me of the time I thought I could beat a hardware store price of $90 for bean bag holes using the Internet plans and then found I used almost $200 in wood and bolts to make my own from the plans! Having said that, this whole project is only justified for home painting, a paint shop may already have a cart!
Don Shillady
Retired Scientist/teen rodder