I started beekeeping last year and really enjoy it but do worry about everyone jumping on the honeybee bandwagon when all of our bees are struggling. I think that it would be great if we also tried to help the solitary bee and bumble bee populations.
I don't like the idea of plastic nest boxes, as mochasidamo says I'd worry about condesation. My western red cedar hive contains quite a bit of moisture from the bees driving the water content out of the honey and I'm sure moisture plays a large part in colony problems. Mesh floors seem to help with the airflow and keeping the moisture content down. There's also the ecological problem of using plastics over woods.
Mikey P, have a look
here at making your own solitary bee house. It's cheap and often you can use bits and bobs already lying around in your garden.
Also for bumble bees you should have a look
here.
Hope this is of help.
ATB
Robin