What about some sort of switch under the battery that closes when the battery is lifted up and sets off a loud siren or horn?
Or install the battery somewhere inside (vented of course). Could even leave the old battery box in the A-frame complete with the old +/- cables inside to fool a thief. I moved our battery inside where there was a perfect spot available for it not usable for much else.
Otherwise, there are lots of lockable boxes that'd work. Some use tool boxes for the bed of a truck. There are "trailer tongue boxes" that will also work and have angled sides to match the A-frame angle and come in steel, aluminum or plastic and in various dimensions. Might want to make sure a box can't also be taken along with a battery as they are not cheap. IIRC, Tractor Supply has a selection of boxes that could work and you wouldn't need to order one online somewhere (shipping is costly due to size). Lowes has some too I think. A battery disconnect switch would be a good idea (or just disconnect at the battery) or solar.
I bought one of the aluminum tongue boxes the same as in the photo and was also going to cut out the angle channels supporting the battery so I could lower the box. But we got a new TT and the front cap bulged out more and the box wouldn't fit and had to sell it.
