1. You can't see how much water you have left. When it's on your back you do have a bit of an idea that there isn't much weight there anymore.
2. Refilling is a pain.
Fitting a water bottle cage fills up most of her frame (Cotic Soul XS), there's definitely no room for the Ocelot as well. So looking at various sites I drew up a template as they suggest and did some measuring. Only one problem, not cost but time. She's wanting to use whatever we'd come up with for the Cairngorms Loop in three weeks' time. Not long enough to get a commercial custom frame bag. Make one ourselves?
Then I had a thought. I've an Alpkit Fuel Pod (medium size) that's meant to be a top tube bag but it looked like it would fit. Strapping it to the seat tube with the taller end by the bottom bracket it was a perfect fit!

So not MYOG more "bodge your own gear" or "repurpose your own gear". I'll add a photo this evening.