behind the seat hydration

I've used an XLab behind the system for years. I have a new one that fits the seatpost for my P3C quite well. 

The cages are identical for frame vs. behind the seat.

Back when I set mine up, two behind the seat was all the rage.  Now it's one behind the seat or none, and a between the aerobars setup.  What I use these days is a between-the-bars Speedfill (http://invisciddesign.com/Z4.htm) and my XLab behind-the-seat two cage setup.  The right cage holds my Gatorade, the left cage holds my spare tire and tube and stuff rubber banded to it, and the Speedfill is water.

I think I have a spare XLab behind-the-seat holder that bolts to your seat. If you want it, PM me your address and I'll mail it to you. No cages included.

Gotta experiment with what works.

I used the DSW set up pictured above for a couple seasons, tucked in tight, but had some issues that negated the aero benefits.  First, the angle of the cage would hold a regular water bottle fine, but the plastic store bought bottles that they hand out on course would tend to eject in bad roads, train tracks, etc.  And this was using gorilla cages.  At NOLA 70.3 I lost one bottle and there was not another aid station for 20+ miles, came back to haunt me on the run big time.  Plus you risk a penalty if you choose not to pick it up (or don't even know it's gone).  Second, due to my fit I could reach back and get the bottle while in aero, but no way I could put it back in without sitting up.  Even sitting up, trying to get it back in behind me the bottle would sometimes collapse, and I'd be sitting up way to long dealing with it.  So it didn't really work for me.  The extra material like in the pic may have solved one issue but not the other.

This year I went back to the x lab double bottle set up (I only run one cage on it for 70.3 as it's just water I get at aid stations)  Not as aero, but bottles don't eject and I can take the bottle out and put back in while still in aero (although losing some benefit by reaching back, not like sitting up).   Now run two bottles as I am going to be IM training.

I have all my flat kit on the bike in a modified Torhans frame bottle

Also have used metal cages BTS but one year at Widlflower got of the bike and the whole thing had just broken off due to metal fatigue, probably from trying to make it tighter.  Use gorilla cages I've picked up over the years