Mounting bat house on cedar siding - too heavy?
Nice work with those mounting brackets you made.  I think your basic plan is totally sound.
Ideally you'll want to locate studs and mount those brackets on studs, and drill through the siding and sheathing and into the studs.  I think three 4&quot; x 1/4&quot; lag screws on each bracket would be good, there's enough meat to lag screws that size to hold the weight of the bathouse.
Rather than seal those brackets to the siding, I'd put a thick washer between the brackets and the siding.  I'd rather have that little gap for the siding to get wet and dry out.  That's just my opinion.
I see one possible problem with the design.  The french cleat will leave space between the bathouse and the siding.
The bats won't know which crevice you intended for them to take up residence, and roost between the siding and the bathouse.  You might want to add to or modify your design to close off that space.