Looks pretty good, ...one thing though I'd try to get a strap for your fire extinguisher.
I'm sure it's held in tightly by the cage yes but if you were to have an accident the last thing you want is to be hit in the head by a flying 500gram-1kg missile. It's up to you obviously but I would just for safety.
You can buy them from eBay, Demon Tweeks etc depending on the size of your fire extinguisher but you could also make one yourself cheaply enough from say some tough velcro strapping as you're not needing it to be FIA approved.
As long as it goes all the way around the extinguisher & under the cage it would do the job.
Link: http://www.ebay.co.uk/itm/Reusable-Coloured-Velcro-Hook-Loop-Strap-Straps-Cable-Ties-Packs-Of-10-/311550264437?var=&hash=item4889d7c475:m:mN12ep380eL1XBV918YhV0A