Permanent Adhesive LEXAN to ALUMINUM ??

I have .045" clearance to the next fin. By design rivets expand so I am limited to screws. I imagine the thickness of the heatsink fin to be roughly .020" This imparts a little head-scratching thread-holding wise. Not even a RIVNUT fits. Also musing the design if I overlap the fin, the end caps become a little dicey to seal...eh? 300 watts is going to broadcast a lot of heat radiation. I could make the space taller.

If a kid with a slingshot can fracture .250" Lexan, he would have already been employed by one of the cartels.