Here is another way to carry the 10x AA battery pack for your LED mountain bike lamp. A small camera bag with the inside measurements of 95 x 60 x 30 mm is the perfect size to hold the 10x AA battery pack.   The camera bag costs between 5 € (Real) and 6.99 € at Promarkt.  "Hama" has the "syscase II" 40H or the  DF 12, which will work perfectly for the purpose of strapping your battery pack under the handle bar stem. 

Here shown is the camera bag battery pack strapped to the handle bar stem and connected to the Tubelight

You can strap the camera pack to the handle bar stem with a 15" (380 mm) Velcro "One-wrap" 19 mm (3/4") wide (minimum) double sided band.   To make mounting even easier, rivet the Velcro strap to the belt loop on the camera bag.

You can also buy separate "hook" strip and "loop" strip and cut each in 8" pieces and rivet them together and then rivet the strap to the case belt loop.

The battery holder is sensitive to corrosion so you should take measures to protect the contacts and batteries from rusting.

      
Lightly oil all the metal parts on the battery holder.    Lightly oil the ends of the batteries.       Use inner tube strips to hold the batteries in place.

     
Use an inner tube strip to hold the 9V clip in place.      Get a silica gel pack desiccant.         Wrap the battery pack and desiccant in plastic wrap.

The desiccant can be removed now and then and baked at 90 deg. C for two hours to regenerate it.