you should be ok putting an extinguisher securely anywhere you like in the car, however, within easy reach is the best idea (making the boot not ideal) Glovebox, cable tied to seat adjuster etc?
Doesn't have to be screwed in place either.
Fuel is just one of those things; can't be helped.
You should be able to sign on up to around 11am, although you may have to wait for a briefing. This is always at the organisers discretion but the RAF are very helpful in these circumstances and high on common sense

Hope this helps