I suggest you get with a good carpenter about what is doable. What is the current window made of?

You’re best bet is to order them from same manufacturer that made the windows

I tried to DIY mullions, and I broke the window! I used tiny tabs of adhesive to hold them against the window and tacked the edges against the window frame with teeny tiny nails. It looked great for about a week, and then we had a bitterly cold night followed by a bright sunny morning and that east facing window had a crack that stretched from one side to the other! I suspect I chipped the glass under the trim or otherwise compromised it. It was a $2000 mistake, and needless to say, the new window has built-in mullions.