I did come across the Magtank and it looks really nicely thought out, but at $100 or so here in Canada it is too expensive for how small it is. Also, not waterproof. Here on the West Coast it rains a lot, so you need some water resistance/waterproofing or your gear is going to end up wet.
I would not want larger on my top tube. As well and as a former backpacker, I never trust any bag to be water proof, anything crucial goes into a baggie. Here’s a pic of my Topstone, you can get a feel for the size.
