Well I guess I better finish this post off..
To mount the box to the top of the outback draws I simply added 4 bits of stainless sheet angle to each corner..
I used 5mm counter sunk bolts which anchored threw the slides, and 6mm bolts where it cleared..
Carpeted and the in she goes.
After thinking 250mm wide was maybe too much I'm glad I went it..
It fills up quite quickly when you have to put everything in it..