Drill some holes and rivet those puppies on. You can see I'm using duct tape to hold the extrusions in place while I drill the holes. You don't need to use washers on the inside for these rivets, they will do a very strong job of gripping just the ply. Yes, I was surprised by this too, but I learnt this from a real flightcase maker. If you want to prove it to yourself, get a scrap bit of ply and rivet a scrap bit of L extrusion on. There is no way in hell you're pulling that L extrusion off.
[pic 2] is the box so far, now for the corner pieces. Elcom part 1064 from this page
are the ones I used. I put the "feet" part of them at the rear of the case, so when the handle is at the top, it's sitting on the feet.
Hold a corner onto the case and drill the holes out [pic 3]. I didn't use a pilot hole, the corner piece held the drill bit in place. IMPORTANT:
For the holes that are going through the locking extrusion, make sure you only drill through one side of the extrusion and not all the way through! The rivet will grip just fine through one side only.
Rivet on all the corners and step back and admire your work. Looking very sharp indeed!!