Help with project

Hello. I am trying to create this: "36&criteria=bobbin+saver
except in a straight version, not round. I have a piece of 1/8-inch
diameter silicone tubing and a piece of 1/4-inch diameter silicone tubing.
My question is: Is there a way to calculate how wide I should cut the slit/opening so that the piece will grip different sizes?
Any information about this will be appreciated.
Regards, Amy
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