Is this the Novara bag from REI that you tried?

http://www.rei.com/product/780453

If so, I'm not sure what you did wrong. Because this fits my brooks just fine.

If you liked that Topeak bag and still want to use it, then this device is needed to make it work with Brooks saddles.

http://www.topeak.com/products/Xtras/F33Fixer

Otherwise, as one poster already suggested, you need a bag with straps. Which may be the easiest solution.