Thought this may interest some, I have adjusted a WAECO WCI-13 13 litre icebox to allow for use of the seatbelt clips on the back seat to retain the icebox and stop coppers bitching about an unsecured load...but mainly so that the wife and I don't get smacked behind the ear by an esky when braking.
The straps I used to clip into the seatbelt are to be had on ebay for the princely sum of AU $2.18 each, the side release clips can also be had on ebay for a pittance.
https://www.ebay.com.au/itm/Pet-Dog-cat-Car-Safety-Leash-Seat-Belt-Harness-Clip-Adjustable-Puppy-supplies/202115753011?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
https://www.ebay.com.au/itm/4-Tan-GhillieTex-IRR-20mm-Military-Side-Release-Buckle-NSN-8315-99-497-9272/183000488494?hash=item2a9badda2e:g:bnQAAOxyrM5THZ0k
Note that there is no stitching involved and the job takes about 20 minutes by yourself and 2 hours if your number one daughter is helping.
regards.