Triathlon Bike Accessories Under Saddle Bottle Holder Dual Water Bottle Cage Mount - Behind Seat Mount Only (Cages NOT Included) https://a.co/d/9AdrI39
This is what I use. It’s true that it will eject bottles. Learned the hard way. I use some lezyne cages that are unnecessarily tight now no more issues
I use this for a 4 1L bottle set up. works well