Kev, Please find attached a couple of pic's of the bracket i made to carry my rear inner tube, note that the bracket on the wall has rounded edges were the one that's on the bike has been left as that's what i wanted, although the inner tube is relatively light around 70% of the weight is lying against the tail of the bike with the bracket holding & supporting it in place, it doesnt obscure the OEM brake light and the bracket is strong enough to support Tyre levers etc, additional brake light etc can also be mounted to the bracket as i will be doing in future.
I finished it of by adding a couple of luggage brackets (Hein Gericke number plate bracket) one either side to hook into with a small cargo net, i have driven around extensively with no movement of the tube whatsoever, i added a safety factor by drilling holes for a cable tie for off road abuse
I am the Operations Manager for a Stainless Steel fabricator and as such i can produce these if there was enough interest.
I had this made as i was able to carry everything in my Tail pack and Bar bag except the Back tube as it takes up more room and hence the need for this, let me know what you think.
Cheers
Steve

PS Hope these photos upload okay.