The crossbrace can be constructed in various ways, depending on the overall construction order you settle on (this needs planning out with any cab, but supers especially).. There is athread somewhere covering different approaches members have used.

Mine were made of a single horizontal section, rebated top and bottom to accept the upper and lower parts of the vertical section.. This way you'll find the upper and lower sections will actually slide into place after the horizontal is fitted. 

There isn't really any way of doing it that allows every joint to be screwed, one will have to be just glued, but if you do it the way I suggest, put the top one in first and screw from below, then the bottom piece canb be screwed from the bottom panel and the pressure from the horizontal section above it should be enough to make a good joint if it's a good tight fit...