I used a vise-grip dimple tool as I recall. I have some "nutplate" rivets I use in tight spots like that - this is a good example of where I use them.
They are cherry 3/32 rivets (https://www.aircraftspruce.com/catal...erryRivets.php
) and work just fine for holding nutplates.
There is also a pop-rivet dimple tool that uses a nail to pull dimple dies together. I've used it a handful of times, but not all that impressed.