get yourself some 2mm steel sheet cut it to shape leaving a reasonable amount of metal over the edge of the hole, tack it in place then weld it into place in short sections( so as not to distort the metal too much) until its completed welded in without any gaps in it then cover it in a [!Expletive Deleted!] load of underseal on both sides,
thats what id do anyway, infact ive done it.
id say if youve got a welder and you sort of ish no how to weld give it a good go, then grind off all the messy bits and then underseal will make it look an ok job wen it comes to mot time,
as far as i know (which i dont know a lot, if you were to pop rivet it i dont think i would pass an mot, but i really dont know, someone else will put you wright on that, hope that helps, and makes some sort of sense
matt