Hi mate, yes I've done this job and to be honest it wasn't that painful.
I wouldn't bother fabricating your own part unless you have all the kit and know how to use it. The rad support is basically a flat panel with a top hat section welded to it, but IRC, there are doglegs at either end which will make fabrication more awkward.
I bought a pattern part off ebay for @ £50 with the intention of chopping the old support off complete and fitting the new one in its place. That didn't happen.
Once I'd got the spoiler, centre panel (bolt on job) and Rad out of the way I could see that the pattern part would cause problems if I chopped out and welded on. The XJ-S part has chamfered corners. The pattern didn't, which meant it would foul the front corner panels if I fitted it...
My solution: The upper (flat)section was rust free, so I just ground off the rusty lower section, drilled off the the lower section of the repair panel and reshaped the ends and button welded the new top hat to the flat panel I had left in situ.
While the box section was accessible, I removed any scabs and painted the inner bits with Hammerite. Once welding was complete I sprayed copious amounts of Waxoyl into the box section.
Whole job took a couple of days.
Tel