can be any shape you want
is it internal?
use timber, plasterboard, flexi arch skim bead...
if youre really clever you'll make up your arch frame on the floor working off a line drawn on the floor the radius of 1/2 your span... if you want a shallow arch just increase the radius and it'll start higher up than floor level... be a good idea to draw it out on the floor before you start building so she can see what she's gonna get...
if youre a bit sh't with joinery get a chippie to knock it up...
good tip is to use 2 or 3 strips of 40x10 timber (ex 2" x 1/2", just thin bendy stuff) for the inside of the arch, if you pin it down to an 8x4 sheet of ply you can bend it to the exact radius you want using just 3 points, side - centre - side. pin it down and build your framework to it. Then just build another one on top of it so its exactly the same, cut some more 2x2 to whatever length you require to separate them to the required reveal width, join it altogether, pick it up and fix it in the opening. clad it with board, bead it and skim it...
