>Could also be used to force the bees to build straight comb. Any one tried it?
Plastic has the least reputation for inducing straight comb. Bees will often go to great lengths to not draw on it making a total mess.
But yes, you can put foundation in top bars. With wax, the problem is the bees clustering on it will often cause it to come out. With plastic perhaps you can get it attached well enough that it doesn't fall. But that won't necessarily get you straight comb.