In article ,
"Swingman" wrote:

"dadiOH" wrote

2. 2-4 pieces of white oak about 2 1/2" wide but on edge

The above, only use scrap plywood ... should ultimately be stiffer than
cross grain wood.

Try it ... if it doesn't do the trick, go for angle iron or square tubing.

The easy thing would be to put a couple more casters in the
center...Probably one in the middle of the cart and one at each end.

This will solve the bow in the middle problem and take some weight off
the existing casters. Make sure you get some the swivel easily - so as
you tow the cart around the middle casters don;t lock up.