observation window - rubber vs. duct board

Well folks... I bit the bullet and made seperate inside and outside walls and ceiling rafters. My observation window will now have one pane of glass on each wall. Nothing about the two walls is touching other than the common concrete slab, except for the finish inside the window itself. (Please see sketch.) I'm about to put the inside glass back in but I have the following dilemma: 1) The experts say to use fiberglass duct board as a finsh substrate to connect the two sills. 2) I made two sets of finish pieces, one of 1/8" rubber, the other out of 1" duct board. 3) I have tried both installed... for each I have someone tap on the inside sill with a hammer and I go listen on the outside. 4) The rubber seems slightly better to me in not transferring vibration but it is not in keeping with the experts. your thoughts? Note: in the sketch I did not call out the rubber seat for the glass nor the foam between the glass and the window stop. Creatch

I would think a breatheable option would be better.

I want to make it air tight so that water vapor doesn't get in there and fog up the window. I'm putting absorbative dessicant in the trough of the bituthane flashing; and the substrate (rubber or duct board) has holes on the bottom sill piece so that the dessicant can suck any moisture through the holes.