I bought one of the pre-bent roll bars from the first of AshG's group buys
For the back stays I used some 27mm OD seamless tube ( 3.25mm ) wall .... For the cross bar I used a section of the wishbone bush tubing !!
To be honest I just used what I had kicking about the place
Quite happy with the back stay material but not 100% certain about the choice of material for the cross bar
( Should be strong enough !! )