Please use this information at your own risk.
Sometimes you need a gasket or seal that is either no longer available or not available in the format you are looking for. I was having a hard time locating thicker seals for the two piece rims on our M923 Military truck. Due to rusting the standard 1/4 rubber o-rings just were not holding. I needed something both thicker and wider, something like a silicone band seal. The cost of having those custom made was prohibitive. Using wood scrap and pure 100% silicone (not silicone II) available at any hardware store I was able to build exactly what I wanted and get the truck back on the road again. Check us out on youtube or at our tour truck web page www.nelsonoffroad.com
The top video is the process of making the rings, the bottom installing it in the application.
It is very straight forward just remember, wax the mold or you will NEVER get it out.
Making rings the size I did it took a minimum of 6 days to harden and then another day out of the mold to set up properly. Wait long enough for it to harden then 3 more days. If you are making something with a cross section more than 1/2 inch then all bets are off for drying time.
Work hard to keep bubbles out of the mix. Cut the tip of your tube very fine to get it as deep in the mold as you can and backfill behind yourself letting it dome slightly.
If you have large bubbles in your product you can inject more silicone into them and let that set up again.
Try to use just one tube but I have been successful transitioning from one to another, just make sure both tubes are fresh and try to mix them a little at the joint with a toothpick. Don't introduce air bubbles at the joint.
You might try vibrating the mold with a palm sander to settle the silicone. This is only going to work if it is fresh and fluid and the mold on the small side. On these large molds I picked them up and repeatedly dropped them a few inches, maybe 20 times. I feel it helped.
Baking the molds at 120-150 for a day or two might speed hardening.
Casting companies have pourable rubber silicone for making molds, it might be worth experimenting with that medium.
All bets are off if you are using this material to hold back solvents, gasoline or gear oil. I would recommend a test where you drop a chunk of what you are using into a container of it for a nice long soak test.