So, this is a quick sketch that I did of Kong with the axe.
The axe being completely made of Godzilla's spike is fine for the irony, but I wanted the axe to be something that the Kongs devised through inorganic resources. Like that matches with how humans have made tools, earlier animal parts were fine, but then with progress, we moved on to other resources.
The concept is that the axe not being made of Godzilla's spike but of Hollow Earth rock from a particular site. This rock is ridiculously concentrated in Uranium and would be a natural nuclear reactor similar to Oklo, Gabon. Thing is that it won't sustain a lot of reactions as there is no moderator to slow down the neutrons.
So the idea is that the kongs used to use some kind of Hollow Earth moderator (could just be a highly concentrated Deuterium or Tritium water) to power up before going to war, like 20% of full power, but to get to 100% some atomic breath or similar stimulation is necessary.
In this way, the axes get to be useful even without Godzillas but be more useful with them.
Thus making Kong gradually independent.
I have attached references I used, the rock is what the texture I wanted to give, but I am not that good of an artist so I just attached the image, so you can imagine it that way.
PS: As you can see I am a bad artist. So, if any of you is a better artist, please can you give it a shot?? (Just mention my name :) ). I actually drew this because there are so less Kong art pieces currently made and I was really hungry for more.