If there were a 26mm stem with the same dimensions, I'd buy one. There isn't. Therefore, I'll do this the simple way. A big rat-tail file to start with followed by some sanding to smooth out the work. I can get within 0.2mm of 26, I bet. I'll also bet my life that the kludge will work for at least 20 years without failure. I've modified other aluminum parts before and never had a single problem. That said, I WILL periodically inspect the stem for stress cracking (both outside and inside the clamp). If I see anything that begins to show signs of stress, it's new stem time. Should I accidentally take too much metal off the stem clamp, I can always use an aluminum-can shim (which I've also done before). If I were working with carbon, then yes, the fit would need to be perfect. Aluminum is more forgiving provided it isn't subjected to environmental corrosion and/or exceptional stress. Stems are manufactured with a SIGNIFICANT safety factor (unless they're carbon). 0.6mm isn't going to weaken this part significantly.

Phrased another way, I'm far more comfortable about my safety using a modified aluminum stem than I'd be using a brand-new carbon one. The aluminum has FAR more safety factor built into it. The carbon trades safety factor for light weight.