Very basic but useful item: a thread plug.

I use these to seal off empty holes in tools/machinery to keep cutting fluids, chips, etc...out of the hole. These are 1/2"-13 thread with a slotted head. The thread is a Class 5 which is tighter than a normal thread. I initially made these to seal the empty holes on my Kurt milling machine vise, but the possibilities are endless. Can create size to suit your needs.