I think you may struggle... I looked for a generic plate for my Korum Chair and there was nothing that would fit, other than Korum's own which was extortionately priced... seems all generic plates are built to one size only... even the adjustables
Arry. You could be right, 20mm seems the normal. I've seen one for £89 but its much too much to spend just on a footplate, as much as I want one. I'm told waterline did one, but they seem to have all gone now.
MDI Match Fishing Universal Accessory Arm Rest 23cm (9"). Non-Marking Fitting fits both 25mm, 30mm Round and 20mm Square seat box legs, the large easi-grip tightening knob ensures you get the right grip on your leg.