WOW! How expensive! I have a really old mobile carrier (from the days when mobile phones were humungous.) It was supposed to stop those dangerous ommissions from hitting you! But it has been a poop bag carrier for years now. I think I paid about £2.99 for it.
What about a waterproof drawstring bag like we used to have for our wet swimming stuff many moons ago,could be attached to your waist and easily be opened to add another bag.
I do, and should probably make more use of it! I have 4 small dogs, so the amount of poo on an average walk can easily fit into one, and it does make it nicer not to have an hour of the poo smell wafting along as you walk! :-)
I take a small - very small - rucksack with me and use that. I double bag the offerings and then put them inside a carrier bag that then goes in the rucksack.