Go to your local hobby/craft store, or eBay, and buy some "ShapeLock" or one of the other low-melting-point plastics.
The stuff gets soft at hot coffee temperature, and stays soft until just warm, when it hardens.
I’m now using a LeLuv tube with a rolled entry edge, which doesn’t need modification, but my old tube had a narrow edge and dug in hard. I made a wide base flange with shapelock. If your tube is oversize, you can also fill in the inside of the opening a bit to make it smaller.
I tried making a "tang" that extended out over the scrotum area, but it didn’t work very well. I might not have had the right shape. You’re also not limited to a round opening any more; a simple D shape mght do the trick.
You can just dip the mouth of the tube back into hot water, soften the shapelock, and re-mold it as you wish, pinch off or add bits, etc. It stuck very well to the plastic of my tube.
Hot water doesn’t bother it when cleaning, but a baby-bottle sterilizer will melt it. Oops.