UV glass...

[AndyL]

Basically, the glass in those halogen lights is UV coated, tempered, approximately the right size for DE bulbs and thick enough for the heat produced by the DE lights. Yes one could get a piece of glass cut, but generally the cost for that is more than just the halogen light fixture itself.

Take the glass, toss the rest - or retrofit the DE bulb into the fixture, your choice...

One doesn't necessarily need UV coated glass, as standard 1/4" will filter out enough.