Wow, i didn't catch that part of "Notdigital's" post. Herman is correct. The acetic acid in MOST silicone is corrosive. If you were to seal the rear section of the shunts up, you might want to seek out some non acidic silicone or apply standard silicone and let it fully cure before installing them on the preamp. You can tell when it is fully cured because the smell is MUCH less pungent.
I really don't think that this step is necessary though, as the RCA's would normally be "open" and exposed to anything floating in the air as it is. Just putting the "shunts" on and covering the soldered area with the supplied caps should be more than enough. Since the shunts would basically seal the internals of the RCA plug, the only thing left to corrode or collect dust / dirt would be the "shorting strap" inside the shunt. At $3 for a package of 8, i think that we can afford to replace them once in a blue moon.
While i have never made "caps", i would assume that you could buy the same RCA's and simply ( and carefully ) pull the center pin out. Fold the ground lug over to the other side of RCA "ring" and solder away. While not nearly as glamorous or "high grade metal" as the Cardas pieces, it should be just as effective IF you fill in all of the rear gaps with solder. Hope this helps.... Sean