Forgot to mention:
The silicone I use: Magikmold GT-6142 LINK
I have used several brands and this is simply high quality silicone. Everyone has their own brand so not going debate anything. Use what you use.
The real key is vacuuming (degassing) the silicone. I have this system:
I'll say that no matter what brand silicone you use, degassing is HIGHLY advised. It only takes one air bubble to ruin a mold and you are out quite a bit on material and time. This is just me of course.
Just an example of the degassing action.
Onto our project. So far so good.
Before I pour the plug, I use some masking tape around the edge. Then a light bead of hot glue. I like having a slight lip on the plug and this keeps it sealed.
Can't stress enough how important it is at this stage to use rubber to rubber mold release. Quite a liberal coat and I usually go over 3 times to make sure.
I use older silicone scraps as filler to reduce the amount of silicone needed for the plug. On the sidewalls and outside I don't skimp, but here is no issue.
Fresh poured. It's leveling slowly and a few bubbles coming out.
Tomorrow we shall see what kind of mess we have made.