In reply to maybe_si:
You could just use araldite/epoxy glue if you can be bothered. The hold in Tom's Roof is held on with araldite and was glued decades ago so pretty good stuff. Alternatively use something like Sika which is also very strong.
However not sure how important that hold is anyway. Isn't the most straightforward sequence to go from the little RH pinch/layaway straight to the small LH flat undercut? That bypasses the RH undercut altogether I think.
The undercut is useful is linking In Brine into Sardine as I remember but hardly an important link up.
Also I wouldn't worry too much about making a mess. The shattered limestone isn't the most aesthetic of rock types so unless you're excessively careless it probably won't show. Put a bit more on than you need. Push the hold in place and wipe the excess off with an old towel.
Post edited at 17:12