Before you say put a yoke in it, I don't have one laying around. Do those red plugs they sell work, or is a yoke really the best way? I plan to put the engine and trans in the car together so I will be tipping the assembly quite a bit. Trying to avoid a mess if I can!
Most of the plugs I have ever seen are just thin plastic dust caps. If there is an interference fit thick walled rubber plug that has some plunge to it, then that would certainly work as well, if not better than a yoke. You might be able to find PVC pipe that is the correct ID/OD and then put a cap on the end, and smooth the open end use that and secure it with strap of cloth fastened to the case on both sides.
They absolutely do work. I've used them many times. https://www.classictransmissionsolu...0-700r4-c4-powerglide-2004r-output-yoke-seal/ These are pretty universal in fit. This one is like the one I bought many years ago, https://www.summitracing.com/parts/pow-pow301230
