Good day Harrier. I have done a bit of resin casting. I use a RTV (Room Temperature Vulcanizing) rubber for the master. Two part, just add part "a" to part "b", mix and pour. As to the master, I have only done stuff like packs & veh storage. I glue the master on a piece of plactic card and build a box around it. I have found a 1/4" clearance all around works well. I then pour the RTV into the mold. A bit at a time and I brush it onto the master. Fill the rest of the void. I move the mold around, tilting it it to allow any air bulbs to rise to the surface. Once cured "unmold" and allow to fully air cure. Mix your resin (part a & b) and gently pour into mold (reduces air bubbles). once hardened gently flex mold around sides to release the new part. Again allow too finish air curing. Remember the RTV usually has a long "work" time , while the resin can have a very short "work" time so you must be organized. As to brands, I have no idea what is available in your neck of the woods. The molds usually last for a couple of dozen castings. Find RTV rubber that is flexible and resists tearing. Long windened but hope it helps.
Doug