Permanent Adhesive LEXAN to ALUMINUM ??

AL heatsink black anodized
300 watts LED
Expected plastic temp max 110c (BTU radiation)
Fins are spaced .045". Rivets contraindicated
Airtight imperative sea shore environment
Convection cooling (Yrchhh!)
The "air tight" part is what worries me.
The power supply will be remote (30 meters)
Lexan for little boys with big slingshots

My first choice would be 3M's VHB structural adhesive tape. Make sure the surfaces are very clean and free of debris, roughing is not required as VHB uses molecular bonding, a smooth surface is better.

If the panels come apart, it won't be at the bond.

They do make a glazing specific formulation - perhaps it offers more UV resistance than the non glazing type?

ALU frame
Or
ALU sheet

My experience with ALU is scrub with scotch bright
Rinse with vinegar wash , paint or glue or whatever as soon as dry aka within minutes
Liquid nails usually works for me
I haven't tried it with lexan
The last window and frame I made with ALU and lexan was 20 yrs ago
I drilled holes thru the ALU and lexan and riveted it together and riveted in place
Then sealed frame edges and rivet holes with 3m marine sealant
That window went thru many thunderstorms and never leaked