I don't need all those tool boxes, but how's that cup holder working out for you? I've been trying to come up with something like that, without drilling into the fender.
Look in boating stores. Like West marine and or overtons online stores. They have many things that clamp to rails for boats. MOstare for the 3/4 or 1" size rail if i remembe right, i think 7/8" is a popular size. If i remember right our rails are smaller than that, but maybe not. Id look at what they have they may have a clamp on drink holder or you can buy clamps, if they are too big of a rail size you can fabricate bushings for it so that it tightens down.