Hello Allen & All,
I  am thinking of making a solar melter.
>
> I understand that the design is nothing particularly difficult, but I
wonder if
> there is any necessity to use glass on top, or if plastic will work as
well --
> assuming it does not melt.

I have seen both used but glass the most on solar wax melters. . The most
clever solar wax melter I ever saw was a old refrigerator laid on its back
and wheels put under it. The beekeeper turned the melter to the south east
in the morning. At lunch he turned the melter to the south west.  The
beekeeper rendered all his comb from a 2000 hive operation in two of these.
He even built a special cement pad to keep the melters on.
Bob
Ps. complete deep hive bodies would fit inside the refrigerator solar wax
melter.