Selftapping screws are a big nono. Alistair recommends SS rivnuts, his procedure can be found in one of the last club mags I produced. If you don't have these issues I can look up his mail and repost here. He can sell you complete fitting packs for the undertray.
Alternatively the undertrays can be fastened with rubber nuts easily. I ordered these from Ebay in different lengths. Longer ones when there is room as in the bumper, the shorter ones in the sandwich floor (just in the lower floor and foam, not through the upper floor).
As an example:
http://cgi.ebay.co.uk/M5-Rubber-Well-Nu ... 3cab751754 I use these every where I can in my Midas.
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Very simple and about an hour work. First I positioned the undertray with duct tape, a few pieces of wood supported by a jack. Then I drilled small holes for temporary fit with selftapping screws and marker for the bigger holes. Removed undertray, carefully drilled bigger holes for the rubber nuts, installed rubber nuts and bolted undertray into position. I used SS allen key bolts with large washers under the head.
The coupe does not have a sandwich floor which can complicate things. I never found a good solution for the coupe. It is a single floor that seems to be too thick for rivnuts or rubber nuts. I once had the idea to use a strip of steel and weld nuts to it. This would be fitted at the inside in the cabin, so the bolts would go through the floor to pick up the nuts. But I never got to the practice.