The disposible food containers made by Glad and the others work well.
On most of the square ones, the sides are tapered, depending on the size of your board it will (may) suspend the lens above the bottom.
The round ones work nicely for non-mounted lenses. Plus, they are light weight, and cheap.

Grocery stores are also a good source for slip-on caps, just find one that fits, and buy the can of whatever it's on, leave it in the food pantry collection box if it's not on your "food" list. (though probably not rear caps for most LF lenses)