Maybe too late now, but I’d recommend using closed end pop rivets, and to back this up an adhesive rubber sealing button, especially for any roof rivets.  The buttons go over the hole on the outside you then just push the rivet thru to button and it gets compressed down, that with the closed end rivets gives you a waterproof seal.

I was thinking of dipping the rivets on marine sealant just like we do with kayaks here but got too excited and forgot LOL, I’ll do it for the rest of the walls, Also I still need to paint the exterior so I’m hoping that the paint will form some kind of seal, we’ll see