I was just thinking out loud and violated the idea "think before speaking". I found some other information below. We're good to go with T11.
From the South Carolina Dept of Natural Resources.
Rough sawn, cypress, pine or cedar lumbers are good choices as a nest box building material. If smooth or dressed lumber is used, be sure to tack a piece of mesh screening to the inside under the entrance hole. This will enable day-old ducklings to climb up and exit with the brood when the hen calls from the water below. All nest boxes should be checked each winter prior to the nesting season to replace nesting material, check the tightness of the predator guard and secure the lid and screen door latch. Production inspections should be made as often as possible during March through June, but at least once in April or May.
Specifications for Wood Duck Box Construction
Materials
- Rough cut cedar, yellow pine or cypress boards 1" thick are preferred, but exterior plywood is an acceptable substitute. DO NOT use treated lumber.
- Weather resistant latch on the side flap that opens for maintenance (ex. 2" zinc hook & eye)
- Zinc coated 1 1/2" inch wood screws