Straps and more straps. Transits have securement bolts spaced along each wall, as you probably have noticed. Can't always strap from one side directly to the other, so you may have to make an X across the freight with two straps. With no E-track, you have to secure skids from slamming forward. Use a strap across the front of the pallet and secure to floor bolts. Don't strap high across the front of the freight stack as it may cause damage to freight. Or run strap through back of pallet and secure so it can't slide forward. Sometimes a length of 2 by 4 placed on floor between freight and wheel-well prevents shifting but then you have a potential hazard of flying wood if in a wreck/emergency stop.
Multiple straps, multiple lengths, rated for heavy loads, at least one long enough to hook on a forklift to pull pallet out from front of cargo area, and one long enough to go from floor over a tall stack and to other floor. Seriously consider getting e-track. That's my 2 cents. I'm sure others can offer better ideas as well.
Good luck.