As you can't use the "strappy" packs with this set up, what's the best pack to buy ??

I think you're limited to those that fit onto the bracket supplied. I don't have a saddle bag for my I-beam setup, so can't advise specifics. I can't see anything online either... hmmm, contact Silverfish?

Topeak

Yup - topeak,or erm topeak it is I have subsequently found out !