Yes HDPE plastic is extremely hard to bond as it is highly chemical resistance. There one product out that can do it but way too expensive even a regular mower shop to obtain. Now big factories either sonic weld these tank or use this hazardous chemical that they can get fairly reasonable priced due to the pure volume they would be using it in.
As Speedy suggested, plastic welding does work as long as you use the same plastic and don't over heat it. Over heating of the plastic is the main reason for welds failing as it makes the plastic brittle, but it would much easier to find a good used tank or purchase a new one. For those of us with shops it is also a liability issue as well, for if your welds don't hold and things do happen then you can be sued for damages.
Of all things don't use an open flame even around an empty tank. Just don't want the boom over here.
