Rivets (making the hatch wider)

When I built my sidewalls, without thinking, I made my trailer a little wider than 4', so now while building the hatch I see that I have a problem. My aluminum left over for the hatch is exactly 4' wide, so to be able to install the corner profile I need it to to make it a little wider, so I'll use a 1/8" thick Aluminum flat bar on each side of the hatch like in the drawing bellow.
I have two questions:
1. What kind of rivets should I use so the whole thing is watertight?
2. Any other solution to this? :-)

I have two questions:
1. What kind of rivets should I use so the whole thing is watertight?
2. Any other solution to this? :-)
