Bit of follow up.... I cut a section from a a 5/8" (I think..) bolt and ground it into a sort of rounded triangle cross section. Dremel'd & sanded a channel in a piece of scrap maple, cut a piece of leftover rubber webbing to line the channel. Ideally the 'dimple-bit' would've been tacked to the c-clamp, but I don't weld & my biking pal who does is enjoying warmer climes at the moment, so I used a bit of double stick to (kinda) keep it in position. Basically used a bunch of clamps to hold everything in place and then slowly tightened down. Seems to have done the trick. Looking more closely at the bikes I have that are factory-dimpled I probably cudda/shudda made the dimple-bit a little longer, maybe.. Didn't need to do much- I just wanted another couple millimeters clearance on each side for 35c tires. One frame down, one to go- but the 'one to go' is built up and I was too lazy to drop the wheel & take the crank off today.