i'v got the same problem with mine , you need a military roll hoop . they come up sometimes on ebay , frogisland list them sometimes .
i had a word with my m.o.t tester and he would be happy for me to fit them ontop of the middle bulkhead provided they are secure , i think series three's had them fitted there so it is acceptable , would only be a stop gap repair until i get a roll hoop tho .