You can use any kind of strap. The boxes are not going to jump off the frame anyway because they are so heavy and the front of the trailer does not bounce that much. The main thing the straps do is hold on the cover. No need to fasten the boxes to the frame any other way.
That is the way my trailer was set up until a couple of weeks ago. As don0128 mentioned in his post above, here is what I am using now. :) Here it is ready to mount on to my trailer after I hook the two batteries together.
Torklift battery box.Barney

