I have tried cardboard tubes and  wooden blocks and have found the natural reeds work the best for both mason and leaf cutter bees.

I have staggered the length of some reeds so not all are 6 inches, that is the minimum length.

They are also sized for Mason Bees in the 8 mm range so the bee can choose the reed size they want. 

Though I have had Leaf Cutter bees use this size as well on occasion.

FYI 100 Mason Bee reeds weigh 12.4 ounces roughly.