I got one of these too, on Amazon. I got this one. It seems pretty good for the money, but I haven't used it fully loaded yet. It was cheaper when I bought it but $26 isn't bad. For OP, somebody on Youtube reviewed a bunch of these bags and liked them. Good way to see them before buying. I think they used a lot of cheap Amazon bags.
I actually ordered the Blackburn medium bag and the RockBros together and opted for the RockBros. It’s subtle and looks good and I just keep tools in there that I might need for the road.
It’s this one: ROCKBROS Bike Frame Bag Waterproof Bike Triangle Bag Bicycle Under Top Tube Bag Corner Pouch Storage Bag for Cycling Accessories https://www.amazon.com/dp/B07QM4X22Z/ref=cm_sw_r_cp_api_i_0M2DT4SSDB336XPJEGYG?_encoding=UTF8&psc=1
I only had to buy two things for it, the total build was like 30 minutes plus some wire routing:
https://www.amazon.com/Woodpool-Connector-Connecting-Batteries-Equalization/dp/B0B9XMS8MG
https://www.amazon.com/ROCK-BROS-Resistant-Triangle-Accessories/dp/B07QM4X22Z/