There's actually multiple ways you can create this ingot, and that is by combining either iron ingots, catherium ingots, or aluminium ingots with...

Depending on the complexity of the ingot that you put in, you get a different yield.

So aluminium ingots, for instance, will yield more fixite ingots rather than if you just used iron ingots.

The description for fixite ingots is that it's synthesized by fixit to be the perfect construction material.