RE: Fusing in a Microwave Kiln
The nature of glass in its liquid form is to round out. If you think about a drop of water sitting on a flat surface, you can try all you want but you are not going to be able to coax that liquid into a square:) Its natural tendency is to form a round drop — glass is the same.
You can try reducing the firing time, but glass fusing really works best with careful monitoring during firing. That’s not something you can achieve with a microwave kiln, unfortunately.