Equestrian Saddle Pommel Water Bottle Holders.
I made these because of my own need. 

I think they're pretty handy, they'll adjust to your water bottle size,  shown are all 32oz bottles.

Adjustable for any size water bottle.

They're light, easy to attach to your saddle with pommel straps.

Price differences depends on your choice, some of the leathers may vary in price. The Blue buffalo is the higher price.