I added two right angle metal strips to the top of my A-Frame between the Propane Tanks and the Trailer wall.
This is what my three batteries looks like sitting inside the angle strips... I bolted the battery cases to my angle strips in lieu of the strap you mentioned

When I added four batteries to my battery bank I replaced those angles with longer units so I could mount my four battery cases which the two outside batteries extend out some over both sides. I could have just added longer strips on top of the ones I added but decided to go ahead and replace them.
This also allowed me to drill most of my holes into the added angle strip with only one bolt into the top of the A-FRAME.
I thought about welding these on but of course the large amount of heat might do more structure damage then drilling...
I'm glad I didn't weld anything as things keep changing. I am now wanting to install one of those 48-inch Truck Side mount boxes to install four Trojan T105 6V batteries in to go across my A-FRAME so I add some security from thief for my batteries...

Roy Ken