I don't remember which size nail I used, as it was an a variety pack with
all sorts of tacks and stuff. Glue the nail in first with shitloads of the
silicon sealant, and then you can fiddle arount with the cartridge holder part
to get the correct length. The pipe parts are 3/4 inch. If you bring
a cartridge with you to the store you can find the parts easier. A friend
made one with an "L" shaped top part instead, and it works fine too.
When the whippit comes out, it is VERY VERY cold. Don't get your hands anywherenear the stuff, and don't use METAL pipe for a dispenser or you will burn