You say mobo and psu is proteted against humidity? how?
because the way how air circulate under the table and some instalation gadgets
the best proof is that all of my PSU's still work fine,
after summer and winter true tests
Hot air being lighter than cool air, it create under the table a pocket of hot air protecting your hardware from humidity & such, i guess you didn't put it in a too much windy place
For the insect life i bet it would be enough to just have sides to keep the hot air, and a fine grill under to close up the rig space. Should also keep rodents out