Just go to any store and look for items that are packaged on a piece of cardboard with a vacuum-formed clear plastic covering (You know the type. Almost everything is packaged that way nowadays)
Find something whose plastic cover is the size and shape you're looking for (Maybe a cheap toy that one of the kids will enjoy! You'll get your covers, and be a hero with the kids at the same time!
)
Those plastic covers even have mounting flanges for mounting them to the cardboard!