I got one of the new super soaker waterguns (http://www.hasbro.com/nerf/en_US/supersoaker/) with the water magazine, but it ran out quickly. This is my solution, i used my water backpack and replaced the mouthpeice with a tube to go into my watergun. This is my first instructable if there is anything i can do better let me know.
Step 1: Materials
3" of 1/2" cpvc or pex (i used cpvc)
A 1/2" pex barb threaded male adapter
A 1/2" to (whatever size tubing your water backpack has) threaded female adapter
a (whatever size tubing your water backpack has) threaded male barb adapter
A heat gun(for the pex)
A cuttable block of wood or plastic
A powerdrill with a 7/8" bit
A chop saw
A table saw
A water backpack (like in the picture)
a hot glue gun
A strap of some kind
Step 2: The Wood Block
Take your wood block and cut it so it is 3x1" and then drill a 7/8" hole about 2cm from the end. Make sure the block fits and the hole lines up with the magazine adapter (circled in the picture) you may need to sand the block to get it to fit.
Step 3: The Adapter
Cut the pex to about 2 1/4".Take the heat gun and heat it up so you can put the 1/2" barb in it and let it cool. Now put the 1/2" to (whatever size tubing your water backpack has) female adapter on the 1/2" barb and the male barb adapter on the other end.
Step 4: Putting It Together
Slide the pex into the wood block as far as you can, you may need to sand it but make sure it is a tight fit. Put the pex over the magazine adapter and push the wood block down as far as you can and hot glue it in place. put a screw in the top so the top of the screw is even with the top of the pex, i would do this by hand. OPTIONAL: pit a second screw on the bottom to hold a strap in place. when you are done it should look like the picture. attach it all up and you are ready to go.