Homebrew PCBs

Ok, shame on me, I broke the glass in my UV box.  Too much vacuum.
We've checked a few local places, the glass they have has UV blockers
on it.  I've got more places to try, but I thought I'd ask...

What else can be used besides glass, which will still pass UV without
bending or blocking it?  Something less brittle, more flexible, but
still stiff enough to mostly hold it's shape (my UV box has a foam
gasket along the outside edge of a piece of plywood, the glass rests
on it and is sucked down onto the board).

Even rubber-like flexibility is OK, I suppose - I can make a wooden
frame to hold the edges and use an even thinner gasket, but that's
more work, and it means I have to cut up my films into individual
sections so I don't bend or crease them from the shrink-wrap effect.

DJ