After creating DIY Velcro band, I got to clean my room and found a bounce card look-a-like. It was from an empty box with lots of small cardboard. So I figured, just cover it with white paper to create a bounce card. And it was the same size as my flash (almost).

First, cut out the cardboard into the shape of the flash. Then cover it with a white paper. After that, attach a Velcro tape to the cardboard. You can staple or glue it to the cardboard. At the end, you can attach with the Velcro band for my flash.

Notice that I used staples again, but facing outwards to avoid scratches. And it fits, kinda, well with my Velcro band and adjustable by height.