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Well, it is a bit small... Maybe you could mention that in the description so people can check and resize as necessary.
I can only fit an old (broken) pair of Sennheiser CX300 earphones, and admittedly it takes a bit (too much) fiddling to get the jack to fit in there.
Tried another 4 pairs and either the jack was too big, the earpiece body didn't fit, or the cables coming off of the earpiece wouldn't fit through the bottom part of the cable cutout.
I think an update to the design that includes a secondary hollow for the headphone jack (on the bottom?) would help alleviate some of the problems. Also maybe specifying the intended dimensions of the headphones in your design would help people work out the percentage scaling they would need to use for their own needs.
Saying all that, for a set of headphones that DOES actually fit, this is a really nice design. Great job and thanks for sharing!