They make tanks that have straps that hold them down.
Some have flat straps and some have round straps.
Norwesco has tanks with round rod that hold them down.
Ace Roto Mold has tanks with flat straps.
don't know what your using to spray with but i have a 65 gal tank and on a long day that's not enough. And iam spraying a 1/4 gal per th.
search poly tanks or try sprayers.com and look there
Good luck Charles Cue