I can confirm that size M6 cap head bolts work fine. 25mm is about the right length. 20mm also works if you want to go a little shorter.
I don't have a 3D printer but I'm sure someone could easily print a "grip" to thread onto the bolt or something to make this more "finished"
I will probably just put on a rubber washer and plasti-dip the bolt head a couple of times to build it up and make it a little more comfortable.
get 50 of different length for $19. They are advertised as grade 12.9 which apparently is a good grade of tensile strength...but of course made in China YMMV
ht tps://www.amazon.ca/gp/product/B07P3RDYLL/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
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