Just a little info for you.

Glass has some peculiar properties.  When light passes through it, the
wavelength is lengthened slightly to the infrared side of the spectrum -
heat.  The other, more important property, is it tends to block infrared
wavelengths, thereby keeping heat in the melter box.  I've heard that
plastic used for cooking bags works for such things.  At least it doesn't
melt.  Let us know if it works. :-)

Mark

If what you're doing seems too hard...,
You're probably doing it wrong. :-)