Concrete nest boxes

Hi, I've been reading your posts with interest, having searched on google for how to make woodcrete nest boxes. I look after a small wood in the Yorkshire Dales which I manage for nature, but have had almost all of the wooden boxes damaged by woodpeckers. I put the metal plates on the front, but they just hack around them or in the sides or underneath, so have decided concrete of some form is the way forward. I like the schweigler box I bought, but far too expensive to replace all of the boxes I have up with them. I tried making a woodcrete box earlier in the year, but I think I got the mix wrong, so will have a go using your recipe. Just out of interest, what's the drying time?
By the way excellent designs and great photos.
Neil