Tried the method on #4 here? https://bikepacking.com/gear/bikepacking-hacks/
Else PlanetX Podsacks or Alpkit Koala might do the trick? Been some time since I’ve bought so not sure on what they cost anymore.
Else many LBS’s may well stock the apidura/ortlieb/altura sets, seen them crop up. Admirably apidura/ortlieb aren’t budget, but may be available in stores.