Waterproof rear-rack bags are pretty much a solved problem with Ortlieb Back-Roller panniers. However, they're overkill for shorter journeys where you don't need the capacity, as they add width.
So, I'm looking for a bag that goes on top of the rack. However, it must be waterproof, not just water-resistant.
The commercial waterproof options are just too expensive (I.E. The Ortlieb Dry-Pack, or Ortlieb Trunk-Bag). And to be honest, they're not even particularly special for the price they're asking – they don't even have any internal organisation such as Velcro dividers to stop items moving around (unlike the non-waterproof "Zefal Z Traveler 40" does for example).
So, I'm considering just using a conventional drybag instead. I can get an Ocean Pack 20 litre drybag for £11.49 and the mounting points used for the straps could be used to secure it to the rack with Velco One-Wrap, or just use webbing.
https://www.amazon.co.uk/gp/product/B0D4ZGCLXK/
The issue is, the Ocean Packs also have no internal padded dividers to stop items moving around. If it were used for bikepacking, you could fill the drybag with a quilt/sleeping bag that would pad it out, but that's not what I'm using it for.
The closest I've found is perhaps using padded camera inserts inside to stop everything moving around, but as with all hobbyist equipment, items with certain keywords such as "gravel" or "camera" are overpriced. I.E.
https://www.amazon.co.uk/dp/B0D3TG8SV1/
And even then you need to find one with the exact dimensions you're looking for.
What else can be used inside drybags to stop items moving around? At the moment, I think the simplest and best solution may be to just wrap the items in a towel and stuff it in the drybag, which would stop things moving around too much and provide padding.
