caster, castor

Caster or caster means one of the wheels or swivels on the bottom of a piece of furniture or equipment--
We can't move this; it has a broken caster.
Stop pushing, the caster is stuck on something.
Caster may also mean someone or something that casts, meaning to throw or fling--
He is an expert caster; you can learn a lot about fishing from him.

Castor or castor means a beaver--the small furry creature that builds a dam. Castor may also mean the pelt of a beaver, or the oil obtained from beavers for use as a medicine or toiletry--
The children looked pale, so she gave them some castor oil and put them to bed early.
Castor oil is still used to make medicines and perfumes.

Now that you know that, you can say--
"The lid from a bottle of castor oil was just the right size for a caster, so she put it under the table leg."

Note--"cast or" ( two words ) means a cast--a throw, the actors in a play, or plaster left to harden in a particular shape--considered with some other alternative ( or )--
Are you going to cast or are you just going to stand there?
Are you a member of the cast or crew?
Is that a cast or just a bandage on your arm?