There are threads on this regularly on the Slowtwitch forum, so I thought I’d share (and archive) my own tip on how to make an anti-ejection bottle cage for a behind the seat carrier. I’ve lost my fair share of bottles, and this is the first thing that really seems to hold strong.
Step 1. Get a Trek Batcage. You’ll need to go a Trek store, since they don’t sell them online (Trek policy).
Step 2: Drill holes in the end of each “finger.” I think it was a 3/16 drillbit, but don’t hold me to that
Step 3: Get an elastic shoelace (I used a Yankz lace for mine) and thread it through the holes
Step 4: Pull semi snug and tie off at each end
Step 5: Cut and burn the ends to keep from fraying
Step 6: Go ride a bumpy road to field test.