I work as a solar installer and consultant on the Garden Route and frequently drill small holes in clients’ roofs in order to mount solar equipment and feed cables into the house. I used to a use silicone gun for the initial waterproofing and sealing but found the silicone didn’t always penetrate and tended to squish out, making a mess.
The solution I came up with was to mix roughly equal parts by volume of silicone and corn flour. The first stage of mixing is done in a plastic tub until the mixture starts to bind together like a bread dough mix. Then, using my hands, I roll the mixture into a ball (you’ll know the mixture is right if it forms a soft ball that sticks to an upturned hand for a second or two). You can also add a few drops of food dye to colour the putty before mixing.
You now have silicone putty that can be forced into gaps with a setting time of about an hour.
So far, I have used it to insulate frayed cable, make a door stop, put a rubber grip on a hacksaw, repair Meccano and LEGO, replace a chess piece and drop-proof a phone.