I normally measure my belts 'by eye' H, using the thicker foil, as I think I've mentioned before. But, if you want to set a measurement, the webbing straps were approximately 2 inches wide, or 50mm, so, in 1/48th scale, they'd be roughly 1mm wide (a smidgin over), which is close enough. You can either paint on the buckles (the shoulder adjusters), or add a square of more foil and paint accordingly. For the fasterners, it's tricky to explain without showing a pic or drawing (I'll post some tomorrow), but basically you'd cut the foil at the end of the belt to the shape of the fastener and bend to shape. The P51 harness, like most U.S. aircraft of the period, used what's known as a 'lever latch' fastener or 'buckle', which was a simple locking device. I'll see if I can find one in all the cr*p in my house and photograph it. If not, I'll draw a pic and find a photo showing at least part of one somewhere.