A block of fir 4x8 with holes drilled from both sides in staggered patterns.
Holes 5/16" seem best at 3/4" spacing and 6" deep
Mostly filled Apr/May 2001
And more recently: clic for larger pic
Left pic is the same block in 2005, reused continuously w/ no attention - middle is 'old' and 'new' styles under an open-sided shed roof that covers the worm bin - right is another new condo on a porch shelf, that also shows how readily they nest in any available hole that's close to the right size, the round things being some old pallet jack rollers!
Notice the round hole in one cell of the condo, just above and right of center, which is the result of parasitic wasp activity - bad some years, almost none other years.