You came here seeking expertise from people who work with RTV, and Paulster has shared his expertise with a good answer.

But what you have failed to realize is that your gasket conditions are impossible to achieve with any homogeneous material. What I mean by "homogeneous material" is that it is made of one substance through and through with no layers, no voids, no bubbles and no structure except a continuous solid.

There is no homogeneous material that can be compressed 6:1 (from 3mm down to 0.5mm) and still retain its shape unless you are dealing with pressures found only in stars or atomic bombs. No can do!

If your project really calls for this much compression, you have to use a material with microscopic air pockets or bubbles. Cork, which is full of air pockets, can be compressed about 2:1. Foamed polymers such as foam rubber, polyurethane foam or silicone foam and similar can be compressed 6:1 if they are so designed. Open-cell foams will leak air. Closed-cell foams are nearly air tight, especially when compressed.

I suggest you look for a 3mm closed-cell neoprene foam sheet and cut it with a razor blade or scissors. If you want a digital solution, try a Cricut machine. Neoprene foam is the material used to make diving wetsuits. If you can find neoprene foam sheets described as "low durometer" that means "softer and more compressible."