Following photos at cosplay.com, I'm putting together a medigun for my Medic for TF2 cosplay.
This is a WIP photo. There are more accurate replicas made out of cardboard and foam, but this looked easier. It's heavy though. In theory you can recharge it, but I've read that the power doesn't last long for the payoff.
Still needs to be painted and I have to glue the black piece at the end for greater stability (right now it's in "check for fit" mode). I also have to connect the hose from the back of the gun to the backpack, which will be challenging since the backpack is made of cardboard and foam.
I might add some cosmetic bits on as well. Not sure yet.