I am going to get an old brass channel out and grind it into shape haha those without a grinder will have to use a hack saw, my trad work is that good I get better than these results all day long using under swag but am very keen to see how it works.
With regards to cutting the rubber, I always have about 2mm either end of my ungers but I would think with this it would be best cutting it to a level end with the squeegee, kind of straight along the angle, as you will want to point of the squeegee right onto the edges.
IMO
