blueman wrote:

I would like to glue a 7" wide extension strip of 1/8" PLEXIGLASS to a
POLYETHYLENE window well cover.
snip

Never going to happen, but you can still get a seal between the materials.

Rough up both surfaces with a 36 grit right angle sander to provide a
rough surface, then apply 3M 5300 caulk.

Allow 1 week to cure.

Lew