schedule H is the fire extinguisher rules
http://www.camsmanual.com.au/pdf/10_gen_re...dule_H_Q310.pdf
1.1 Hand-held fire extinguishers:
(i) must remain restrained under an acceleration of 25G;
(ii) must be capable of removal by the driver (or crew, where applicable) without the aid of tools.
(iii) must be inspected by scrutineers at least once every six months, or otherwise prior to competition. This
involves visually checking the unit and its mountings for damage, checking the pressure of the contents, and
shaking the container to check for settling of the extinguishant. Where practical the extinguisher should be
weighed.
simply, the scruitineer is saying that they don't beleive a self tapper will meet requirement (i).
is there some reason you can't drill right through where you have the self tappers and use a bolt with large washers to hold it instead?