I use a "large" changing bag, purchased a few years ago from Freestyle Photo, I don't recall the exact dimensions.
I put the camera into the bag with the side-opening lid pointed up, then zip up the bag. I hold the camera and bag either on my forelegs while seated, or pinched between my legs. Then I insert arms, remove the lid and stow it vertically along the side of the camera facing away from me. Then I have room to pull out the exposed paper, stow it in its slot and insert a fresh sheet. Reverse the procedure to close it up.
Yes, a changing tent would give you more room; but then you have to figure the setup and tear down time involving the tent itself, which only makes the operation more lengthy. A large changing bag seems to be a good compromise between complexity, cost and convenience.
~Joe