So I ordered the only glue that I could find "locally" that promised a good bond with LSE (low surface energy) materials.
In preparation I cleaned the whole tank with a powerwasher, and then used a dremel tool to gauge out the cracks in the tank as deep as I had the guts to go (as my tank was not leaking yet. So far so good I thought...
The glue had a very harsh chemical smell, which seemed promising to me as usually the worse it smells the better it works. Anyway, when applying the glue to the tank at first all seemed well, but then it started reacting with the HDPE and it bubbled up really bad. Also in my test setup, glueing two sheets of PTFE together, had more or less the same reaction and after 24h of drying it didn't hold at all.
So I'm scrapping the idea of trying to fix my tank. Luckily in my country a lot of R33's were parted out, and as I've come to find out the tank cracking issue is less of an issue here due our climate most of them are still intact. I found one cheap, so I'm picking it up soon to replace it with.
If anyone is still looking for an uncracked tank, I might be able to source one here for you in Europe. Send me a PB!