MG rear wheel arch spats

[grogee]

Does anyone have any technical drawings/info on how the rear wheel arch spats are fixed to the body? My MG has gone in for welding/paint but I've got to put the spats back on myself. I think from memory they were self tappers straight into sill but this feels like a bad idea from a rust point of view.

I've seen plastic square grommet nuts that accept a self tapper type screw but all the ones I've seen need a square hole and I'm not sure how to cut that in the sill which is a 'blind' panel.

Any ideas?

[Mr bump]

The spats had a fixing built into the inside arch lip, that always rot.
the base/long section along the cill is fixed by a plug pin mounded into the spat and a self tapping screw.
so when you look at the spat when it's against the rear arch, you don't see a fixing.