Prof Wonmug wrote:
We have a very old appliance that has an electrical cord with no
strain relief. The cord comes out of a square hole in the back. I have
replaced the original cord with one that it more heavy-duty, but it
still eventually shorts out. I'm going to go get a piece of stiff
tubing about 1" long that will fit into the hole and run the cord
through that. Is there a better idea?

There is a thing called 'black electrical tape'. Wrap the cord so that
it fits tight in the hole. A knot in the inside will also provide strain
relief. Or buy a plastic cord clamp that fits the hole.