I have glued QTY 4 AR black glass 8.5" OD circles into the 9" ID of the pcal red anodized viewport guillotines.  All 4 are with Robert now for testing. 

I used the Araldite 2012 2-part epoxy which is routinely used to glue the glass fiber stock to the aluminum holder during fiber pulling production.  This glue seemed a much better choice than the EP30 which is used for many in-vac applications.  I followed the assembly procedure for the Araldite which the fiber lab has had good experience with, namely a ~30min 60 deg C bake on a hot plate.