bit of mix and match mate...those arch kits, same as what christ used but the b.gypsum version comes with a pvc liner what fits over the metal edge...
outer arch has the eml cut off which would normally be the inside, then snipped down the corner section of eml at 2" intervals from the edge to the bead so i could adjust the radius, plonked 1 quarter either side and just let the pvc bit run between em to give me a line to work to, filled it with render..
inside arch has the corner section snipped off, pvc bead again...that one was nearly size...
fixed the uprights, then the internal arch, then the external arch, just used a tape, that pvc bead really helps to blend the shape in when you bend the bead all out of shape to change the radius...
another half kit would have helped, just to help keep the pvc bead in place cos theres an 8" gap between the two outer arch quads, plastic bein plastic it might move, might not...we'll see..
havent got any bead left for the skim though...used the render to bed it all in and the semi-flexibility of the beading to help even it all out with the trowel pressure...shapes good but ill just have to mess about a bit with the skim to get a tidy edge...
just wondering now if me new (almost unused) flexi/adjustable internal corner trowel might become an adjustable external corner trowel???
ill take another pic when i skim it...