I honestly think Plasti-Mend is just ABS filings dissolved in Acetone. But it's just easier than concocting your own homegrown blend. I assume you could take some ABS pipe and a grinder and make a big pile of shavings. Melt these shavings into some Acetone until you have the consistency of ABS glue.
I have read (maybe it was on the Plasti-Mend website) of people using it and fiberglass screening to repair large tank holes. So it or an acetone ABS gel concoction should work for you as well.
Two bits of unsolicited advice:
1) Layer the repair over a few days. If you slop the glue on too think the ABS on the tank will get soft. It's not the end of the world if it does. You just have to let it sit for a day or so to evaporate the Acetone and harden up.
2) I asked the Plasti-Mend people if I could substitute aluminum window screening over the fiberglass screening as I figured it would be stronger. I don't remember their exact answer. But they basically said they do not recommend the aluminum screening and to only use fiberglass window screening.
