In Bermuda, bluebirds have used a gourd-like plant as a nest site after people had dried and hollowed them out. Another unusual and long lasting nest box made of cement and sawdust can be purchased through catalogs featuring bird paraphernalia. I would like to encourage you to try these designs and report back to us on what worked best for you. Whatever box that you choose your primary concern should be placement of the box in the proper habitat. Secondarily, the box should be made predator free by mounting it on a metal pipe heavily coated with automotive grease.
Larger entrances attract starlings.
If you do paint or stain a nestbox, use a light color and apply it only to the outside of the nesting box. This will keep the inside temperatures as much as 12 to 18 o cooler in the summer than a dark stain. Leaving the nesting box a natural wood color is also acceptable. This helps the fledgling bluebirds exit the box. Do not add a perch to the nesting box.