I did a lot of Seat belt anchorage design and testing some years ago at both MIRA and for Vauxhall at Millbrook. I have seen a 65kg load snap 4x M10 mounting bolts during a 30g 30mph impact. I have seen seats tear out of floor and go through the windscreen with part of the floor panel still bolted to them!!
So don't think a couple of straps through the boot panel will do any good in an accident, if you want to secure it think heavy duty webbing like seat belts as they also do a good job of decelerating objects and bolt those straps in securely with spreader plates on the other side of the metal panels with at least m10 bolts, preferably 12.9's(tensile strength) In the event of a serious impact anything other than this level of securing wont be of benefit and you may aswell just leave it loose in the boot.
HFT, for people with only 2 buttucks!
VooDoo TwoTwo The next chapter.