Let me simplify the highly detailed explanations you have received...
1. Model the slot as a rectangle/loop antenna of fine gauge wire.
2. Model the outside edges of your antenna as another rectangle/loop
antenna.
Fill in the area between the two rectangles that is solid metal with a
criss cross of connecting wires.
3. Create a transmission line to feed the two long edges of the slot at
mid point...
4. Be aware that Mininec engines are going to be a bit iffy with this
kind of structure... But at least you get some ball park figures..

denny / k8do