You could probably just take a piece of 2" wide 1/8" aluminum and glue or screw on some high density polystyrene blocks to act as spacers for the rear. they may not last so using some AL angle would be more durable. If you keep the angle only a few inches high it should only contact the rubber on the tire...but if needed add some rubber to it for padding.
When I made my rack for my pickup, the front wheel axle mounts on amazon are stupid easy to mount, and pulling a front wheel takes literally 30 seconds. Mount the front forks, minus wheel. Put the front wheels somewhere else for the trip Added bonus, no straps to fuck up if you have someone else in charge....you won't lose a bike
hmm if really boucing...on your rear spacers make them high enough to clear the inside of the rim. Drill holes and use a hitch pin to secure rear from bouncing. On front of bar do the same...make vertical plates...1" wide is ok....drill holes and use hitch pin to secure the frame. then you can pull 2 pins and each bike is free. no big bars in the way.
I wouldn't strap them down either, with any luck they won't be there when you arrive. Collect ins. Win-win!
You may laugh and this sounds unorthodox but I usually pad the floor , lay them flat and use old sleeping bags and moving blankets as cushions while stacking on each other . It’s ugly and looks scary but we’ve done this even with Honda 50s and what not when space was at a premium . Plenty of padding and strap the “bundle” to the ground.
I like this... if the padding was high enough it would negate the risk of the rear tires hopping out of their spaces.
Cut up an old bike rack that the front wheels fit in and weld a couple sections to your trailer so you can do the 4 or 5 in a row by 2 rows deep?
Anchor some square tubing vertical uprights to the trailer floor and get a few of something like this? https://hollywoodracks.com/collections/frame-hooks/products/long-locking-ratchet-frame-hook
Easiest option would be to buy a bunch of these to mount parallel with the trailer axle, on the floor: https://1up-usa.com/product/van-tray Get these if you want to make sure they stay in place on bumpy roads, without any straps. A trailer with no sides is the most ideal, and I've seen trailers set up like this on RAGBRAI: https://1up-usa.com/product/roof-rack You can get a variety of lock options all keyed alike if you order them all at the same time. 1UP USA is my favorite bike rack I've ever owned. Edit: You can adjust the 'wheel stop' to not hit the fenders of the bikes you show in the back of your pick-up. I see you may be buying a new trailer anyway. If you want sides to store other gear, get a trailer with stake pockets, where you can remove the sides. I'll see if I can locate a picture of the RAGBRAI SAG van and trailer set-up from someone. They did use a Featherlight Trailer like one of these: https://www.fthr.com/product/utility-trailers/open-utility/