I come back to what I said, I think the soldering will be better, because for me I mainly use the inserts on thin sheet metal 1.5mm to 2.5mm, it's really when you can't do otherwise, moreover, do you have the right pliers? because if this is present you have some on "radiospare" at not too expensive, but in my opinion the soldering will hold more than well the 40Kg especially with 4xM8 or even 4xM10 !!