Yup, a leather case with pin back sounds possible. If you make the case tight enough it might cause damage to your kilt while removing the flask from the case (horrors!) so if I were making it I'd probably stitch a small strap with a glove snap on the end to go over one side of the top of the flask---that way it is secrure but can be removed without stress to the fabric. I am actually in the process of carving a teeny cover with a saltire on it for a one-ouncer just like this.

BTW the flask itself I am using (after removing the excess key rings) weighs 1.2 ounces empty, 2.1 ounces full, and I estimate the total, once covered in leather, finished and stitched and filled, to be about 2.6 ounces.

Moosedog