Thanks everyone!
@zendog. Yes, placement is important but so is timing. I’m placing it under the eave of a shed so it should be in the shade from about noon on, and it is white, which I can also cover with a piece of aluminet. Since it is put out early - when fruit trees are blooming, warmth is an asset initially. Probably good to monitor this, though. I have a thermometer which telemeters data to the house so I can watch all this. I’m more concerned about freezing them than baking them. 
The nesting blocks will be removed from the ‘bucket house’ about 6 weeks after the last release of the bees (if you are staggering releases). In my zone this should be before our days become uber hot - I’m estimating late May/early June. I’ll place them in fine netting in a smaller bucket in the garage such that the wasp predators can’t get access to them. My main concern here is our humidity and the fungal issues that that can favor, so I may bring them in the house.
Exactly!
I would like to get some of them out early enough for pollinating, and, if need be, bring the bucket in at night if we get a late arctic dip. I’m also considering some type of heater, but haven’t thought that through enough. I noticed early in the thread folks were concerned about cold getting their mason bees with weather shifts. So that is where I may need more thought and a plan.