I'm using a MX Lock and Load for just one bike, for 4 or 5 I would look at something else because it's $150 per bike.
IMHO I would use a wooden rail and wheel chocks from Harbor Freight on each side of the trailer. Tie down 3 per side with the rear ends coming into the middle. Their economy chock is $13, add a couple of 2x6's and you can do this without breaking the bank. To protect the wall you can use diamond plate, plywood or anything that can be easily moved (a 2x10 perhaps). I mention the 2x10 because I use those occasioanlly to get the truck unstuck from sand, they are wide enough to support the tires. Only happens once or so per year but they are handy to have on that trip!
Good luck with the project!