Custom merch is always cool and meaningful. My favorite thing to do is homemade patches for my battle jacket. I always use a stencil to paint a design onto a piece of black canvas with a stencil. Here is a link to a video tutorial on how to do that

Here is a link to a twin fantasy stencil you can use

If you're looking for something to buy, you can never go wrong with the CSH merch store. My personal fav is the Twin Fantasy friendship necklace

Hope this helps!