Nice! This looks like an elegant solution. I know I always call out the water bottle mount thing in my reviews... It's great to see an option like this and I've actually come across a couple of similar designs that could work. One challenge with some ebikes like the Neo series from Easy Motion is that the downtube diameter is much larger and might therefore require mounting to the seat post tube with something like this.
For bumpy or off road applications I still prefer a Camelbak because I've had bottles fall out of cages or bounce all over and get my legs wet. I also like those bottle cages that mount under the seat and can hold two drinks. I think they're mostly used on road bikes in races and stuff.
I was actually just chatting with another person about add on water bottle cages for bikes that don't have braze ons or bosses and we found this M-Wave cage on Amazon. It's very similar to the one you've shared but uses a screw clamp instead of velcro straps. For only $8.21 it seems like a good deal and you could probably even mount it to the back side of your seat post (as long as it didn't get in the way of reflectors, racks etc.
Here are a few other neat water bottle holder add-ons I found while exploring. One is basically a six pack suspended from the top tube and there's these cool bar end cup holder things (lots of cup holders out there now that use the handle bars and work for Starbucks and other plastic insulated cups. Another shot of some front mounted cages and finally, these really cool little metal adapters that actually add braze ons to standard bicycle tubing! You just mount them and they have threaded holes built right in.