Making a diabetic-friendly cake is very complicated, we tried for months and could not come up with a recipe with a good enough taste and texture to sell. (Some recipes were OK, but not good enough in our opinion for a product someone pays for.)
Many diabetic recipes we found when doing our research use white flour, which is not really acceptable to a diabetic due to the high glycemic index, so you would need to experiment with low glycemic flours like amaranth, quinoa, or sorghum. I doubt you will be able to put together a cake that will hold up to carving.
When we got requests from diabetic customers, I would share my personal opinion that a tiny piece of excellent non-diabetic cake is preferable to a regular-sized piece of mediocre diabetic cake.