I would load it like you would into any other pin hauler (SUV, Pick up truck). Legs off, coin door towards the interior. In the back of a trailer, you'll want to strap it down at least a little bit as it will want to slide back and forth.

Completely blanket and shrink wrap. Stand it on its back and ratchet strap it to the side walls, bottom to the wall, assuming there are tie downs

I’d just load by hand and if you can get a 2nd person to help all the better. Use a truck to get game to trailer. Take game off truck. Stand game on end facing trailer edge (bottom facing trailer, coin door up). Lean game forward agasint trailer edge. Lift back of game and push on to trailer. Use a large piece of cardboard or moving blanket under the game to help slide on and off

If it's Uhaul, the 5x8 isn't really big enough to stand them up and there aren't any good tie downs at height on that model. Do not trust the wooden slats on the sides for real weight, they will pull right off the wall.

Just slide them in side by side and strap them down to reduce sliding. Put blankets or cardboard in between.

Completely wrapping them is up to you, but overkill for all but the nicest/newest/rarest machines.

Went down this road.

Here is a video of how many and how I got into my U-Haul for a pinball show last year.

Here is a thread with extensive discussion and many pictures. https://pinside.com/pinball/forum/topic/u-haul-trailers-how-many-pins

Okay... specific to your question, I've fit 5 in the back of a 5x8 U-Haul before (see video above).

IF you are hauling 3 or less pins, there is no reason you would have to stand them up on their head once inside. If I were hauling 3, I would get a moving blanket, and put it on the edge of the Uhaul. I would move a pin wrapped in shrink wrap over to the edge of trailer and tip it in, letting it down slowly. From here, I would side the pin on its blanket to the back of the trailer. I would push the pin all the way to the rear of the tralier, and wedge a pool noodle between the rear wall and the pinball machine. I would use ratchet straps to ensure the pin did not slide during rapid accelleration and attach to the anchor points in the rear of the trailer. I'd repeat this process for the other 3 pinball machines, using old boxes and pool noodles to wedge inbetween. If you are hauling more than 3 you would have to stand them up, which is fine. I've used a 5x8 several times to move like this and have never had an issue. They do sometimes offer a 5x8 with a ramp, but I have found them to be much less easy to load than the 5x8 with ledge. I think its actually easier to load them into the trailer than into my car.

Make sure you buy a lock for the back of the U-Haul. Even if you are not worried about something walking off, that rear hatch can open if you don't physically lock it. When I drove home from renting one the first time the roll door was open when I got home even though I know I'd closed and latched it.

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I remember your thread last time, you worked hard to squeeze those into a 5x8.

A 6x12 is a lot easier to load and stand some up, but I think you had a small tow vehicle.

I don't like playing Tetris anymore, so I finally bought a transit van.

This ^

All easily doable by yourself. This is how I do it by myself in my suv or NV3500.