Trouble attaching lid of heater matrix

I have bought a replacement for a leaking heating matrix and cleaned up the box around it. I have put some foam on the inside to try and make it as airtight as possible.

My plan was to rivet the lid back onto the box. I have hit a snag: the holes on the cover are pretty small and only a small rivet will fit through. At the moment my immediate issue is that my rivet gun does not go down that small, so if I need to use these small rivets, I would need to get a smaller rivet gun.

However, I am worried that these rivets may be too small for the job. The holes in the main part of the box are significantly larger, and maybe too large for the rivets to catch on.

So

1. Should I be using rivets to attach the lid, or is something else recommended?
2. If it is rivets, should I persist with the small rivets (and buy another small gun) or should I drill the small holes out and then use a larger rivet.

There's deffo a big difference there ... if it was my box, I would drill out and fit larger rivets.

Alternatively. beacause i'm a big fan, i'd drill an M6 hole on the cover and then drill/tap into the body, either side of the existing holes and fit some M5x8mm dome headed set screws.



Other sellers are available, only use stainless if the bodies material is steel, otherwise use BZP screws/bolts.

Most rivet tools will do the 3 most common sizes. I would do as @miktdish above and drill out for the size you have.

Just original self tappers in mine, if you need to get in again but harder with rivets