If you don't fancy making the bits yourself you can buy the inner and outer skin as repair panels now, the last ones I saw were on ebay.
One tool I found invaluable when doing the bits around the vents is a power file for smoothing out the welds as it gets into the corners much more easily than an angle grinder.
On a quick google search I can't find anything at the minute, I have seen the 'inner clamshell' (I think is how it was described) for sale somewhere though. Might be worth sending an email to Pegasus as they seem to do the vast majority of the bulkhead components:
Thanks, I'll mail them
Now I take a better look at the picture of the one on Ebay mentioned in the thread, I see it's the same iron bar that I had already purchased..
On the YRM-metal solution site they show the outside instead of the inside..