Roy,
someone in the towed gun GB made a rack.
you may want to search the thread.
the 'complete' Quad kit (w/ gun & limber) does not have the rack either.
(i am just finishing mine)
i did not use the rack, i just put the spare on the rear deck as the
instructions showed, and assumed that it was bolted down somehow.
the rack looks like a simple frame made of welded steel,
i found a couple of bad pictures on line.
i will have to see if i can find the links.
this page has good pics of it.
i think they are the 5th, 6th & 7th on the page.
also the last 3.
http://www.dana-nield.com/vimy/FAT.asp
good luck.