You could instead use a "UTH" on the top as well, with a reflector over-top (for ex. a rectangular UTH with a rectangular metal baking pan over it to help deflect heat
downward into the cage). I've done this before with some old ceramic UTH that put out high heat at low wattage...works just fine & no light output. In combination
with UTH from underneath, it should be warm enough...esp. if you insulate the glass part-way, & depending on your ambient room/house temps. as well.