Lowering Down Tube Mounts

[richardfm]

If you are prepared to drill your frame you could fit an M5 x 0.80 rivnut lower down.
A bit of googling will show you how to do it without buying the special tool

 

[freiston]

There are loads of adapters cheap on Aliexpress if you don't mind waiting a few weeks (check for free delivery and be aware that taxes are added at checkout):

https://www.aliexpress.com/w/wholesale-cycling-bottle-holder-adapter.html?catId=0