Can and should one install rivnuts into an aluminium frame? I have an aluminium frame, and would like to add another bottle cage.  But there are no mounts below the downtube. I've installed rivnuts into a steel frame before, which worked well.
Can I install rivnuts into an aluminium frame?  If the thickness governs yes-or-no, then how would you measure the thickness before drilling?

Searches turn up conflicting information, ranging from &quot;OMG no!&quot; through to statements like:

Some have expressed fear of weakening the frame by drilling holes in it.  If done properly with the right kind of rivnut, the rivnut adds strength back to the frame around the hole.  If you use steel rivnuts on an aluminium frame it is actually stronger than the metal it replaces. 
from https://www.electrifybike.com/technical/installing-rivnuts-in-bicycle-frames#/

So, should one install rivnuts onto a frame made from Aluminium?
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I would recommend you not do this.  I have first hand experience with drilling holes in frames, albeit only one hole.  Although it was a success, the situation was different.
The short version...on two separate bikes, I drilled into the seat tube to allow for a dropper post routing.  I have had no trouble.
I did my homework, considered pros and cons, and most importantly considered the cost of ruining the bike.  After drilling, I would ride it and then inspect it, gradually increasing the amount of rigor of the test, to the point that I was going over jumps and landing flat.
Other posts suggested that there could be cracks that I don't see.  It is possible, yes, but based on the amount of abuse this bike has gone through, I feel like if it were going to fail, it would have failed by now.
Your situation is different, you would need two holes, and they are proposed on the downtube, where there is more stress.  If you have a butted frame, the material is thinner in the middle, right where you would want the holes to be.
I do not think the rivnuts will add strength to any holes you drill.  This might be the case if it were a perfect fit, and welded in with no voids.  But that is not the case.  They are not designed to add strength to anything, just to add threads to something that is already plenty strong.
You will not be able to figure out how think the material is without drilling into it, at least not that a layperson can do.  I expect someone scientist with the right xray or ultrasound could do it, but practically speaking, your guess is the best you can do.
I would suggest a similar approach to what another user mentioned.  But here is how I would do it.  Cut a strip of old inner tub, about 2 cm wide.  Wrap it around your frame once or twice where you would have wanted to drill the holes. If you want to glue it on there, you can, but you don't need to.  I'd suggest Barge Cement, just do it outdoors, that stuff has some pretty serious fumes.  Now just zip-tie the water cage on there, nice and tight.   If the zip tie slips of the spot on the water cage where the bolt would go, just bend a little lip into it.
Lastly, I get the hunch you are the kind of guy that likes to tinker with stuff.  Me too.  In this case, I would suggest your tinkering skills with drill bits would be best applied somewhere else.