Please note; I have never done this; so am not talking from personal experience.
However; I believe (and I may be wrong; I often am;) that the principles for completing this job are as follows are as follow.
1. Get a metal fabricator to make up some plates
2. Secure plates to chassis
3. Drill into the plates using 12mm bolts
A local garage may help with the above.
Other folk have drilled 12mm eyebolts into the chassis and then used ratchet straps to hold the tank down...
Good luck.