So, tried this out, and thanks for it! I think I was able to do some expansion, but it was rather tedious. I found a mason jar with a 2.5" diameter opening. I don't have any aquarium tubing at the moment, but instead used my silicone sound that has a hollow bore and a syringe. I couldn't really suck any air out with my mouth, so had to use that.
I feel like the biggest problem though, was by sticking my penis and scrotum in and just sitting normally, there'd be a gap between the top of my penis and the jar. The only way I could fill the seal was by basically sitting on the jar carefully.
Do I need to start with a smaller jar? I seem to have average size balls, and my penis's width is about 1.25-2 inches, depending on how hard I am and such. It's also hard for me to really be erect when doing this, as my penis is long enough that the head pushes against the bottom of the jar and as I get more erect, it pushes on the jar, messing with the seal...
I'd really appreciate any tips!