What a co-inkydink - I'm doing exactly the same thing. Just bought a black HD Tour Pak from FleaBay, and am pondering the mounting options. Just waiting for my new Russell Day long seat to arrive so I can have My Royal Sweetness (Mrs.) perch Herself comfortably and line up the Tour Pak.
I'm hoping to use the Triumph rack as the mounting platform, and wonder if it will support the Pak, contents etc. weight wise. The Pak weighs in at 32 lbs, plus whatever contents, so we need a sturdy mounting set up. The Triumph rack is mounted using the side rails (2 front mounting points), but the HD mounting system is a 4 point set up (2 front and 2 back).
Otherwise I'll have to fabricate something from scratch. I'm not a welder, but willing to learn.
I'd be very interested to see how you end up mounting the Tour Pak. This should add considerably to the comfort and carrying capacity for 2 up touring. Please keep us posted. VinnyBoy