The Busch and Muller bracket at the bottom of your SJS link earlier in the thread would do the job, although it might need a bit of bending or a couple of spacers on the bolts.
Depends on how keen you are to fettle, exchanging the rack for one with a mounting plate looks a simpler option.
Although if the new rack fits well in other respects, it could be worth persevering with it.
Much as I rate Tubus, you could buy one of those and find that, for some daft reason, it doesn't sit well on your bike.