What would be strongest way to attach knife scales?

[12345678910]

Make everything flat first.
You can't clamp it into being flat.

 

[Alex Topfer]

You can't clamp it into being flat.

Depends how big your clamp is....

 

[Rick Marchand]

I'm surprised that no one has mentioned cutler's rivets.

I'm not... lol. I also repair and refurbish knives and lost count of the failed cutlers rivets I've removed. Maybe they make em better now?

 

[Stacy E. Apelt - Bladesmith]

Cutler's rivets are fast and cheap. Neither sounds like strong and durable to me.

But, as I said earlier, properly installed, most any fastening method will hold up, including cutler's rivets.