These are clamps I designed with Fusion 360 to hold the "Parametric altoids storage rack
by wardwouts" https://www.thingiverse.com/thing:212918 To a 1 1/2" industrial sewing table
I used 3/8" for the clamp (the nuts are tight you have to use a pair of large pliers or tap them with something) For the attachment screw I used 4mm nut and a 4mm Flathead screw again tight.
I used a flange/T-nut with a very short set screw glued in the top side for a stop. Depending on the diameter of the t-nut you may have fun getting it to fit in the cap. One was loose the other was tight I glued them both.
2 clamps are needed.