On one of the long gone TTs we had, I used some 1/4" carriage bolts with one size larger fender washers to clear the squared off sections to hold the battery box on the angles. I also made up a wood spacer to keep the battery away from the bolt heads as well as provide better drainage.
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011
Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014
Montana High Country 343RL (38')w/disc brakes