I got some sheets of honeycomb from saxonpc, which is actually not real honeycomb but looks more like glued straws. Only these are way more solid than my own glued straws so I’m pretty happy with this.
Fairly simple: just cut out with a sharp knife, add some “hobbyrubber” to fit it to the flash. I’ve made it so that there’s room in front of the grid to insert a second grid.
Only thing was that the SB28’s diffuser sticks out quite a bit on the top of the flash so I had to make a little cutout in the grid.