Moldmaking Best silicone/resin to use if you don't have a pressure pot?

Anything with a low viscosity and not to short open time. I would recommend SmoothOn Smooth Cast 305, it's very low viscosity and you have a whole 7 minutes to work with it before it starts setting up. SmoothOn resins aren't perfect, but out of all the types I have tried they are the only ones I would readily recommend.

SmoothOn also has an even lower viscosity resin made for slush casting called 300q but it only has 30 seconds of pot life so that doesn't give you much time for anything.