Here is something to try with your PVC tube molds... I've had very good success with it.
On the bottom of the tube is a "test cap" which costs only a few cents each. Here it is shown as I'm unmolding but it fits directly on the bottom of the pvc... though it won't usually stay on by itself.

Over the test cap I place a plastic sandwich bag or a thick piece of plastic cut from a garbage bag.
Over the plastic I place a PVC coupler. This holds everything in place and mostly stops any soap from leaking out. Sometimes a tiny bit leaks out into the plastic bag but not usually.

Here's a better picture of the coupler along with a batch that was just pulled out of the PVC but still wrapped in one of the plastic liners I use.

I know they make actual end caps that have flat bottoms but I have not been able to find them.
The ones at Home Depot and Lowes are rounded so they won't sit flat... but I'm keeping my eyes open. For now this works well and doesn't require a lot of fuss.
Oh and they stand up well in the fridge for the SBW batches!!
