Oh, I misread your statement, you said "hygrometer" not "hydrometer." I guess that would make a difference. I'm not familiar with a lot of pet supply's, but going by the link it is a humidity meter (as well as thermometer). I have a similar produce called a digital weather station just like this one
Acu-Rite Deluxe Wireless Weather Station and Atomic Clock, but I got mine at wal mart for $40. It has a remote censer that works up to 100 feet away from the base station to measure both inside and outside readings. I will also have one of these in the herb greenhouse I'm building (possibly two for accuracies sake).
I think your referring to the sides rather than the bottom. That is you plan to use OSB for the sides as well as the bottom, and want the sides 6 inches tall and 4 feet long. If so ya, they would bow out. But I drew up a drawing of a simple construction that would support the weight, both on the bottom as well as sides.