Your B5 rack (the first one) needs a set of the half-circle clips that are used on the front mounting of the pillion seat (the rear seat mate of your front seat)you would also need to drill a second hole in the rack leg above the mounting rail.
I do not have the bike at work today but when I get a chance will unbolt the rack and take some photos of the mounts.One difference I have noticed is the rear mount on my bike has a slot not a hole.
Ahh... I think I see what you're getting at now, Gremlin:
... I have lengths of black delrin rod, both 1/2 and 3/4 inch......