Undead Pony

I am looking to add a full size spare to my trunk, Shelby Torque Thrusts 18x9 with the G-Force T/A KDW tire. I used to have my trunk mat out and a full size 17" with tire in place of the donut but since then I have replaced rims a few times and a full size 18 will not fit. I was wondering if anyone had any ideas on how to secure the rim so it doesn't slide into the Shaker1000 system or cave in the trunk mat. I few years ago I came across some guys at a show that had GT500s with full size spares, they used foam to keep them in place, I've searched around the internet but can't find any type of foam that was pre-cut or even big enough to fit a rim. Any Ideas?....

Undead Pony

157dB

Extend the OEM spare tire hold down rod with all-thread and feed it thru
the carpet. The ?larger tire diameter? will keep it from caving into the
spare tire depression.
BTW, rim size should not matter if you are keeping the same
revs per mile diameter with the 18" rims...
Wider, yea. But bigger in diameter?
Better have a rear end gear change to steeper gears...