I found a company that makes a patch specifically for the HDPE material of fuel tanks and guarantees success
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I've contact companies that plastic weld and they say there is NO WAY they can weld the fuel tank because it is a PEG woven fiber layer encased below HDPE
With the patch from that company there are very specific instructions for it, they are designed for our style of fuel tank materials, it will work if you follow the instructions. e.g. surface prep (sanding), heating (heat gun), drill holes at the ends of the cracks, etc... must be followed exactly