those plans look good. i wouldn't worry about holes in the back, the heat will come out through the front enough. belly heat rocks!!! i would use thick shelves to help with sagging. the flex should be around 320 watts with those specs assuming 10 watts/ft. i would just use a thermometer in the bottom shelf and one in the top.
i would use a proportional stat like helix and tape th eprobe directly to the flexwatt. this gives you alot of control over temps.


vaughn