When teaching horses to load my bumperpull is blocked with chunks of railroad ties. Two under the front (frame) two under the back. An assortment of boards are used to shim where needed. It is rock solid.
Saddlebag...that is the only way I would ever load a bumperpull not attached to the truck. Basically, you put a foundation under it and made a room. At least that would allow a person to work with a horse and leave the truck free to use if necessary. Of course, by the time I 'd get done making the thing really solid my husband would be sure I never meant that trailer to move again...because I'd still be on the nervous side about doing it.
Personally, I always teach a horse to load with the truck attached…not only for safety reasons, but because horses know the difference (when you're practicing and when you are really going somewhere because the truck is attached).