There are two 1/8" hardwood dowels holding the rib in place, and 600 grit sandpaper attached to the jig.
This is my solution to the issue of working around a clamp while thicknessing and smoothing rib stock. I started by screwing the jig to the bench with the screws countersunk well below the surface. Then I planed the surface flat before attaching 600 grit sandpaper with spray adhesive. I used a piece of aluminum angle bar stock to make a hole drilling template.
The aluminum angle stock is used as a template for drilling the holes in the rib.
The pencil line to the right represents the minimum distance from rib edge to the holes. The holes nearest to the pencil line are from drilling the holes in the rib using the template. In retrospect it would be best for this to be on the opposite end of the jig from the dowels, but I have not changed it yet.
Once the holes are drilled in the rib it can be placed onto the dowels in preparation for thicknessing. The sandpaper increases the friction enough that the rib doesn't tend to moove much while planing or scraping. The dowels add just a little more holding power so the rib really stays put, but can be picked up to measure the thickness. I keep a little dish of water to dampen the dowels and/or holes to swell the wood if the fit starts to loosen up after removing the rib from the jig a few times. I find that the dowels need to be replaced periodically, so I do not have them glued in place. A harder dowel may make sense, but I wanted something friendly to my plane blade, so I could have the dowel protruding the full thickness of the rib.
The rib tends to start to curl after scraping a bit (once it gets hot?) and wants to start lifting off the dowels, so dampening helps swell them and keep it tight. You can also press a scribe into the top of the dowel to mushroom the top out and tighten it up. I think you could also use this method to run the whole jig through a thickness sander on the inside of the rib after the outside is scraped. Would probably only pay off it you were running several jigs through in succession to get the whole lot done quickly. Otherwise it's probably quicker to plane them.
I have done four sets of ribs using this jig, but it has been working well for me. It seemed like an idea worth sharing. Still room for improvement no doubt, but I prefer it over working around a clamp on the rib.