Fitzo's spot on, two things you can do, file a small shallow notch for the gaurd's bottom to lock into, and pin it. If your tight enough on your fit to look good you can just use epoxy for a seal. That type of gaurd is one that I haven't gotten tight enough for my satisfaction to use the solderless methoud so I still solder them and clean the solder up to a nice thin fillet. You can put the pin anywhere, but upper thirds about works good for me.
There are a couple of ways to do a double gaurd with the handle larger than the gaurd area. One is the loveless methoud of sliding the gaurd over the blade and butting it against stops on the handle, the other is the old sheffield methoud of a framed handle. A framed handle is basicly a hidden tange with a frame made to look like a full tange. Put the gaurd on, then the frame with handle slabs. Bruce Evens has a good tutorial of this on his website.