so i just priced out a 4.5" x 4.5" square steel tube. the smallest i can order it online is 2 ft, and its 79 dollars. i think the wood blocks will have to remain for a while longer..
I would just use a piece of aluminum round stock between each caster mounting hole. Some 1" OD should be plenty to space it out with.
its already pretty difficult to roll around in some spots of the shop. i think im going to come up with some sort of handle on the cabinet side and folding table on the other side, and maybe some sort of low profile handle on the back of the cart, just to give me more grabbing options when i have to go down the back hallway.