One other bit of advice. If you use plywood to make them, seal the edges well with Valspar or some other oil-based sealer. I would use 2 coats at least. I simply used oil primer on the ones I made last year and wished I had sealed them well before painting. I'm now having to go back and use thinned Titebond III to fill in some of the cracks that have opened up between the layers.
Nate