The inner arches are from dereks minis.
Just had a look through my pics, and I've not got any really good ones, so I'll go and get some more, and put them up for you.
The rear flaps, are I think, riveted through the flap and then into the plastic arch. Then, where there is no plastic arch, I've got a little mud flap clip, which is kind of like a G-clamp. There's a slit in the mudflap just at the level of the rear valance, and then the clamp slips into it, and tightens up to hold the flap onto the lip of the rear valance. The flap has been pushed up as far as possible, and then some silicon sealant has been put along the top, to seal the top of the flap to the inner wheel arch, this stops crud from going up into the rear valance, and going behind the flap.
With both flaps on, I get hardly any muck on the car now, only areas that get dirty are the sills underneath.
I think these two pics give a pretty good picture of how the flaps are fixed.

Edited by Cerberus, 27 September 2012 - 01:04 PM.