...also, the holes in the ribs are already predrilled to size #19 holes for #8 screws -- too big for 1/8" rivets. You could use bigger rivets though, but harder to squeeze. It's probably easiest to use flush head screws, as suggested.
Martin, not sure why you would say it's not removeable although there isn't much reason to? If you don't use rivets, there is nothing but screws holding the frame in. Also holes are already large. Getting the lower switch panel out is really worse but both are doable. The panel itself comes out very easy.
Now is also a good time to think about access through the center panel if you plan on putting your AHRS or other electrical stuff behind it. Also note that you have to trim the center rib to allow the radio stack to fit.
Also note that all radio trays are not built equal and are not all 6.25 so you may have to trim the center posts a little bit and/or shim a little. This is a GNS430 and a SL40 shown.