These are also referred to as 'pulks'. I no longer use the thing but when I first built one for carrying gear in the winter I took a kid's sled and screwed on some aluminum angles to the bottom, the kind you can get from the hardware store. Then you attach some PVC pipe via cord to a harness or backpack waistbelt. There are various plans on the web for building these things -- I think some boy scout troops even build them for merit badges.
I gave mine to user erykmynn a few years ago.