I used to work for a company that sold polyethlene tanks
that came blank. We would cut a hole and the would use a little handheld machine that we would put the fitting in - I believe it was a Makita -and it would spin in a fitting (or a patch) at high speed and melt the two pieces together.
Depending on where you are I would check with marine
sanitation companies, or tank companies and see if they have something like that. It would take about 10 minutes (once you got the tank to the shop). Other than that it's really hard to stick stuff to that material.