I have had really good luck using Goop to bond patches to Royalex, but you do have to wait a couple of weeks to let it dry. Yes weeks. For larger patches, drill a few 1/16" vent holes in the interior of the patch to let the gasses out, but keep the vents away from anything that was leaking.
I did a test with two clear Lexan sheets so that any changes happening to the Goop were visible. After two weeks I needed a screw driver to get the sheets apart. After a few months they would not come apart.
The article below has pictures and details of how to apply the patch.