Stainless can be slippery, but sometimes that's an advantage.
My transmission build bench is stainless and it works really well, easy to keep clean and takes the constant sliding of heavy parts, plus makes it easy to spin the trans around for the final assembly steps.
I don't use any of the holding fixtures to build with as they are more hassle than they are worth for our setup.
I would tear a plywood top up fast, plus the constant soaking in trans fluid would do bad things to it as well, and it would be a lot harder to keep clean enough for trans parts.
Butcher block would be better, but still not as good as stainless.
On the other hand I guess the main point of buying a harbor freight box is to save money and stainless tops aren't cheap. Even used ones are expensive, and a new one costs as much as the box itself did, or pretty close.
Might be able to have one made, but sheet stainless isn't cheap either, plus having to pay someone to fold and weld it.
Most likely if you have the equipment to bend and weld large pieces of stainless you probably aren't working out of a hf box, though it is possible I suppose.
If you don't work off the top of the box a lot, or not with heavy stuff plywood might be fine, though I don't really see the practical purpose of it. It will protect the box somewhat I guess, but still going to look like a piece of plywood and its going to wear more quickly than anything other than cardboard.
Butcher block would look a lot nicer (in my opinion anyway) and hold up a lot better. I learned on this forum that ikea sells it pretty reasonably. I know they have narrower width stuff pretty cheap, and I think they may have had up to 30" width reasonably priced.
Of course you could glue or bolt a couple of pieces together after ripping one down, or maybe even just set them there next to each other.
What I ended up doing on my older snap on box is getting an old pickup bed mat and cutting it to fit.
Its heavy duty ribbed rubber, probably 5/8" thick where the ribs are and 3/8" in between. I took a little time laying it out and it looks like it belongs there, its pretty tough and easy to clean, don't have to worry about oil getting on it, and cost me $20 from a junkyard and I had enough left over to do another box plus some scraps.
That was from a short bed one, but one for a full size bed would be 8' or pretty close to that so you could do up to a 96" box.