You might try a non-dichroic filter. What is the risk? In 15 years it might fade? Then just replace it again.
Here is a link to high temp. polycarbonate lighting gel designed to be placed over hot theater lighting. For $6.50 USD you get a 21" square sheet. Enough to last a lifetime of printing. Might be worth a try. I believe the fan in the head will be blowing on it, so it might survive pretty well up there.

Also check this catalog for ideas: