Traditional Lionel flat cars can be had for $10, painted with Rustoleum camo, add coffee stirrer wood decks and decals. Deck is a little narrow at scale 8' 9", details are not scale and they ride way too high, but for public displays they are a cheap non-scale option. Besides, everyone will be looking at the Sherman tank riding on the car.
These Lionel cars from 1997 were an improvement but details were not scale and ride height was still high.
Menards makes similar quality non-scale military flat cars. These have simulated wood painted decks and nicely detailed Jeeps or trucks. The forest green color was a definite miss. Also they don't roll well, so a long train can be a problem. OTOH they are ready to run.
Weaver 40' flat cars are a good reasonable cost option. They ride low when converted to Kadees. As Dave C noted, molded on details can be improved.
MTH Railking flat cars are also scale and ride height isn't bad. Decks are scale 9' 3" wide. This one was converted to 2-rail wheels and Kadees. Also available in Premier line with Sherman tanks.
AHM/Pola are nicely scaled cars. Wheels/couplers. Decks were weathered with craft paint. Decks are scale 9' 9" wide fitting the Solido Sherman tank without overhang.
The finely detailed Lionel die cast 40' cars with wood deck look great and have a nice low ride height. You only need to distress and paint or stain the deck. They come with various loads. This one is lettered for Army Transportation Corps and includes a Sherman tank (SKU: 1926751).
Another very nice scale 40' flat car: Chooch 40' Fishbelly Flat car kit. But this fails the "currently available" test as they are hard to find. Also hard-to-find scale kits are the Berkshire Valley 902 O Standard 40'6" Fishbelly Flat Car Kit and the Red Caboose 42' 10" Flat Car kit.
I haven't been able to identify this really nice scale Army transportation Corps flat car I found. It has finely detailed stirrups, grab irons, coupler pockets, stake pockets and under frame. Anyone know who made it?
Bob