It's an older Shimano 600 (or 105, I don't remember) that I had on another bike. 130 BCD with 39T new Shimano ring. I may replace it with a 42T, but I'll need more riding to decide.

I carved a form out of dogwood (very, very hard) and attached it to a C-Clamp. Another piece of wood on the outside formed to match the stay. A few turns and it was done, although the outside shaped clamp pad split on the second one. One side is a bit deeper than the other, but the tires fit with space to spare. I imagine if I opt for wider tires, I may have to tweak it. Here's a pic.



The back side of the block has a groove in it that fits the clamp. This acts as a key to keep it horizontal as I make a crimp. In using, I snugged it up and then counted turns of the clamp screw to keep the dimples about the same size.

It handled the gravel on my street a lot better than the 32's on my commuting bike. On my ride yesterday down a local MUP, I road for a time on the crushed rock "horse/jogger" path along side. I didn't notice much difference (another surprise). On another bike with 25s, there is a definite speed difference between the two surfaces.