Im working on building up the bodywork for my bike engined GP project, as I cant get a seat loop for a good price, incomplete used making £150 and new being £340

what I have no idea about is the standard width of that seat section, ie the distance from the inside edge, to inside edge of the side panels
Im planning on rolling and shaping it, then welding it all up to the side panels to create the 1 piece rear that I want
I have hazarded a guesstimate at around 8", obviously I can make it whatever width, but my whole aim with this build is to end up with something that does actually look like a lambretta, even if theres nothing lambretta except the panels, I dont want it to look obviously "wrong"
many thanks