Traditionally, I and my father before me would use a nail to plug the end of a caulk tube. It worked, but you'd have to deal with rust from the nail and you'd need different sized nails for different holes.
I looked for covers at the home improvement stores, but they're pricey for what you get. I downloaded a couple of options which really didn't seem to fit the bill, so I designed my own.
It's a simple design, and I have it available with and without a plug. I mostly use the cap without the plug, but you might need a plug if you've cut a large opening.
Both variants print without supports, but the plug variant needs a brim on the inside to prevent the plug from dislodging during printing due to the pull of the plastic. The brim on the others is optional, depending on your adhesion.
The cap is designed to sit flat both upright and up-side-down.
My examples were printed in Black Overture PLA Professional (PLA+), though PETG is probably a better choice or possibly even TPU.
If you need or want to modify this item, you can find my Onshape document here:
https://cad.onshape.com/documents/0f9ddfffd2c48ffe6ef832ba/w/f75ee569139fda530f294a1f/e/cfe84519c724155283343020
Please post any makes!
Cheers!
&
Always Be Printing
The author marked this model as their own original creation.