Two of the 10xAA battery holders will fit in a 800 ml water bottle  This bottle will have to be held together with cable ties so it is not so easy to constantly take it apart to charge the batteries.  This design is only practical if you plan on charging the batteries in the bottle.   See charging system.    Electrical tape will not hold over time.   I've tried it. 

With this design, you can drive two tubelights or have a spare battery with you.

1.  Cut the water bottle just under the second groove from the top.  

2. Use the inner-tube rubber bands to keep the batteries in place.

3. Place the battery packs in the bottle and pack bubble wrap around them.

4.  Place the top back on and poke small holes for the cable tie with a 5 mm or so spacing.

5.  Poke similar holes on the opposite side for a second cable tie.

6.   Remove the top and start the cable ties through the bottle holes.

7.  Now feed the cable tie through the hole in the top.   Repeat for the second hole.

8.  Now slide the top over the bottom.

9.  Tighten up the cable tie and cut off the end.  (see picture to left)

10.  Pull the cables through the top.  Pack bubble wrap in the air pockets in the top. Screw on the top. 

You are good to go.  If the cable is tool long, you can pack some of it inside the water bottle.