I have been watching your progress with interest and venture to suggest plastic or even brass tube with a bit cut out. If you do it in short sections it could be held end for end in a vice and the section cut out with a saw.
Ian.
Hi Ian
The thought of cutting a tube down did cross my mind but then I remembered I had some plastic half round stashed away somewhere.
It was duly found and seems to be about 6mm diameter. Originally bought when I first started in 7mm for guttering before I got my eye in!
It's quite a thick section and notably about 20deg plus more than half round. As a result it sits way too high on the ridge. So out came the big nasty file to make it half round...
The edges were then chamfered down with another nasty file and the strip cut into 18" sections. Out came the Mek and... It seems to be impervious to solvent.
So then a trial fix with some superglue, which seems to work.
And then some mortar (filler)...
Might just get away with it?