Onegun's approach is most likely to succeed. The Topeak rack actually assumes you have
eyelets down at the rear above the axle and does not attach to the axle at all. IF you don't
have any eyelets, there is a workaround I have seen recently that involves P clamps on the
seat stay down at the bottom near the rear axle. At the other end, the very short seat tube
on the T900 makes Topeak's rack problematic. In order to get enough clearance behind the
stoker's saddle you will need some kind of extension. I was looking at a Blackburn rack a few
days ago and it came with extenders for the mounts at the saddle end so, as Onegun says
these are available. The mounts assume eyelets on the top of the seat stay, again P clamps
can sometimes be used to simulate eyelets, depending on clearances and manner of frame
construction. Very long extensions on the seat stay end of the mount will make the rack
less rigid, but still adequate for 10# or so. The mounts at the seat tube are slotted flat metal
that has a 90D twist, subbing a length of non-slotted strapping will increase the rigidity. Lowes
and HD have aluminum stock 1/16 thick by 1/2" wide that is easy to deal with.