Other Tips on Nesting Blocks
1. Afix the nest block firmly to its mounting. 
2. Face the nest block to the east or south east to get morning sun
 at least three feet above the ground

3. A large overhanging roof will greatly protect the mudded cells from driving rain and snow.
4. Keep a good supply of mud near the nesting block during the nesting period.  
5. Do not move the blocks during the weeks of active nesting. 
Mason Bees in Reed Grass, ready to emerge in spring.  Reed grass is inherently variable in diameter.  The mason bees don't seem to mind.
Mason bee house with filled tubes inside roof.  These bees will emerge in spring to reproduce and populate the empty tubes below during spring and early summer.
Mason bee house with filled tubes in main body of house.  The cardboard box in the roof contained loose cocoons in spring.  These emerged, and reproduced in the tubes below.
Mason Bees in cardboard tubes.  Notice the color in the mud (mason) varies as the bees work different mud sources throughout the season.
Orchard Mason Bees are impressively flexible about the housing they can inhabit.  Essentially, they just need a hole of about 5/16 inches, with an ideal length of about 6".  The length is not a hard requirement, but the length allows for an optimal ratio of female to male eggs.  You want as many female eggs as possible for best increase of bees.  A few examples in the pictures on this page will give you the idea: 
Mason bee house made of a 6" wood block, drilled with approx 5/16" holes. The required hole spacing is debatable, but people often use 1/2" spacing.  Cleaning these holes is hard unless the house has a removable back. 
Mason bee house made of a 4" wood block, drilled with approx 5/16" holes.  Mason bees will cetainly use this length hole, but it might reduce the ratio of females to males.  You want as many females as possible.   
Mason bee houses with mason bee cartridges installed.    
Mason bee cartridge of holes, with 5/16" diameter, and 6" length. These cartridge layers can be separated to allow removal of the cocoons for cleaning and storage over winter in a cool environment.  The cocoons are then placed in the mason bee houses in the early spring.  (See picture above.)
Orchard
Bees.com