Good morning Ken,
On similar projects I found that drilling a 1/4" deep by 1" dia hole with a forstner bit and then a 1/4" drain below that works well. In the 1" dia hole I insert a piece of the green Scotch Brite non woven material. With its porus body it lets the moisture drain without getting clogged everywhere so water gets out. It also seems to dry out well when its not raining/snowing keeping the seed from molding. Hope that might help.