I had a diabetic dog. She did have to get insulin injections twice a day, but there were many dietary issues involved, also.
I eliminated dry kibble. Most contain grains and also lack moisture. Your cat will need as much moisture/water as you can get it to drink. You may think you should hold back water because of all the peeing, but that is a bad thing to do. They will pee a lot anyway if their blood Sugar is high, because that is how the body tries to flush it from the system. By holding back water when they will pee a lot anyways, this will cause major dehydration.
Minimize/eliminate salt/sodium products. This will cause blood Sugar levels to skyrocket.
Also, check ingredients in all food and treats for sweeteners. I was shocked at how mnay treats, cookies, etc. contained some form of sweetener -- sugar, honey, molasses, maple syrup, fructose, etc.
I'm not sure if glucosamine is a supplement given to cats for joints, but I learned the hard way (through my own research and not from the vet - disappointed he didn't know to tell me) that glucosamine can raise blood Sugar levels.
I didn't feed my dog any grains at all. Really the only carbohydrates she got were lima beans occasionally (her favorite food --- weird, I know). I fed her lean beef or chicken and lots of vegetables. Some people believe they still need grains/carbs, others feel they do not. It's your call, but carbs can turn to sugar easily.