Yes, since posting I have been doing some googling and while I haven't found any published chassis dimensions I have done a CAD mockup, using the external dimensions of 24"(60cm) x 18"(45cm) x 9"(23cm) which are often listed in the specs.
I used CAD to draw up the outline of a cabinet add in the thickness of the sides, top etc add internal corner cleats, the 10" speaker and the sliding shelf and that gave near enough to 12cm(5") as the height for the vertical section of the chassis. This link
http://www.voxshowroom.com/northcoas...15_TE_blk.html gives AC15 cab dimensions where they quote 5 3/16" (13.2cm) as the height for the vertical portion of the chassis.
This link
http://oldamps.weebly.com/vox-ac-10-jmi.html has a picture of a Vox AC10 chassis sitting in an AC15 cab and it looks to be just the right height so I'm assuming my dimension 12cm(5") is a little under size, I should be able to tweak the speaker position and corner cleat size to accommodate a vertical size of 5 3/16" (13.2cm)
Since advising No 1 son that I intend to do an AC10 twin clone he has asked for a master volume to be included and once again using CAD I have come up with a width of 38cm(15") once the master volume is added in, as you say while my build is an AC10 twin clone I can add my own extras.