I found a cloth bookbinding tape on Amazon. It works quite well and comes in several colors. I like the black and grey the best. It works well to cover the edges and gives the look of a matt or frame (I fold it over the edge and stick front and back).
I also use it with stretcher bar frames - I buy standard size stretcher frame bars on Ali Express where the diamond painting covers the front, then the edges wrap and adhere to the frame front and sides. I then cover the sides with the bookbinding tape in a color that coordinates (or black) and it gives it a nice look.
The tape: https://www.amazon.com/gp/product/B077FXJTTS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Here's a pic of a custom photo I did where I did stretcher bars and the tape (grey tape).
It's cloth book binding tape. I use this (not an affiliate link) https://www.amazon.com/gp/product/B077FXJTTS