Gluing plastic is always a bit of a bear.
When I contacted Prijon and asked what I could use to glue to their blow-moulded plastic hulls (everything I had tried had failed), they told me the only thing that would work is "GOOP" that is not available in Australia. I ordered some from the States off eBay and it worked like magic. I now use it on all sorts of things that are hard to glue or need a flexible bond. The bond is strong, but not strong like epoxy, but it stays very flexible. It has worked well re-sealing the foam bulkheads in our plastic Prijons.
I needed more, and found that the same company has a different product called E6000 which the manufacturer told me is essentially the same product (I've found it to be less viscous). It is used in hobby crafts and is readily available in Australia on eBay.
I see that GOOP may now be available in Oz, but looks expensive: https://slhobie.com.au/gear/mounts/485-goop