I decided to go with a water bladder on the trike since water bottles could not be located within easy reach while riding. The questions became: what kind of bladder and where would I put it.
There seems to be all different brands, sizes, and shapes of bladders available. After as much research as I was willing to put in I selected the Camelbak 70 ounce Lumbar Reservoir. I selected the lumbar style because it seemed to fit better across the back of the seat. The 70 ounce size was selected because it holds about the equivalent of 2 large water bottles (28 ounces each).
Commercially available bladder bags did not seem to fit my needs, so I elected to make one that worked in the space I had available. I used a heavy duty nylon canvas, similar to Cordura, some edging, some webbing, and a plastic snap. The materials were all in stock at the local fabric store, and the cost was about $15. I was able to use a home sewing machine, with a #16 jean needle, even thought the material was quite thick.
The hole in the bag was sized so that the round plastic bladder opening just barely fits. There is a small flap on the tongue of the bag that slips over the plastic extension on the bladder opening. When closed up there is a natural opening for the tube to extend out through, and the entire package can be closed up and carried by the strap.
Tube location was an issue so I constructed 2 clips from some name badge clips and short strips of double sided Velcro. When wrapped loosely around the bladder tube they can be positioned at the best location. I place one on the back of the seat and the other on my shirt or jacket.
It is a tight fit, but the bladder bag slips in between the Radical Design panniers and just behind the headrest support post. The bottom of the bag rests on the mounting straps for the rear rack and the bag strap goes over the headrest tube for added support.
I added a quick disconnect in the bladder hose near the bag. This allows me to disconnect the hose and fill the bag without removing it.
All in all, the bladder works very well, and it was a good choice for my needs. Two water bottle racks and 28 ounce bottles weigh 11.6 ounces; the bladder/bag/clips weigh 14.4 ounces. This is not much of a weight penalty for the benefits. The biggest problem I have encountered is trying to standing up without removing the hose clip from my shirt. Ah, when did my memory and my body go in separate directions?