Nice box!

I did my first couple of grows exactly the same way.. See my links below. One item I'd point out is watch your plant height, you'll run out of overhead room much faster than you think. Once you flip your lights to 12/12, expect some stretch. As far as preflowers.. if your plants are 6-8 weeks from seed, just flip them, you'll find the males quick enough. If you want to wait to take clones, there's plenty of pics out there that will show you what specifically to look for.

In regard to your filter... ask for "hardware cloth" when you're picking up your filter parts. I did a similar one inside of a rubber made container I use for drying, though I draw through it instead of pushing the air out. In regard to your humidity comment earlier, you'll find if the relative humidity gets above 60% your filter will be less effective. Happens on Can filters too..

Peace, Farmer Rich