This was one of those projects that makes me love 3D printing! I was recently making some sleep/eye masks and wanted to make the elastic adjustable to fit a variety of people. I had some 1 inch foldover elastic but needed some hardware to make it work, so I just designed it! They printed and worked like a charm and look awesome in different colours to match the fabric, or with different plate patterns (I used a carbon fibre style plate). The hole is designed to fit 1" elastic and if you are making something from scratch, you will need one of each part. The single hole is smooth on the inside while the two-hole part (the real adjuster) has small teeth to offer resistance. You can see the photo for general assembly but if you need more clarification, I can make a more detailed picture. Searching the internet or digging through your laundry pile for something to refer to can also help! I have not tested it on anything besides the elastic, but I imagine these can also work for 1" webbing or similar. Also, you can size the Y down to 75% for ¾" elastic but that may be the smallest you'd want to go with this one.

Sidenote: I used PLA not thinking about the washing and drying of the product- but I have run them through a couple loads to test them with hot/normal temperature water and a low temp tumble dry and they have lasted so far. If this is a concern, I would suggest PETG or ABS to withstand high temperatures better.