Back in 2014 Ken Meyer wrote and article on how to make an offhand rack. He was not using fixed amo and the rifle when in the rack slopes to ward the muzzle. What methods are used to keep the powder in the case when inserted into the chamber. I am given to understand that using a wad can lead to ringing the chamber.

Floral foam in  the case mouth while loading will not hurt anything.  I would tilt the rifle up before firing as Frank suggests.

I use a cork wad when loading a rifle on the "Offhand rack". It goes just in the mouth of the case. I tip the cartridge down at the mouth to get powder to set toward the front of the case. This prevents the neck wrinkling that I get if I don't do it that way. I have found no problems doing this. It does seem to take a tenth of a grain more powder to get the usual accuracy when tipping the mouth of the case down prior to loading. When shooting off of the "offhand rack" I usually use cases loaded with powder and wad rather than reloading the same case over and over. I do, however, still breech seat my bullets.