Give up on computer grow box?

So I was builiding an old server (size = extended ATX) computer case into a micro grow cabinet.
This is my first year growing and now I just plan to grow in my backyard (it's legal here) and put seedlings in my cabinet.

I ran into an issue this year where the seedlings were dieing. Either from low heat or low humidity.
I saved them by moving my LED light onto a sealed rubbermaid bin and now the plants are happy although I need to air them out every few days to prevent mold etc. It was sealed up tighter so the humidity and heat were in happy regions.

Upgrading my computer case to having near airtight sealing is going to be hard so I was thinking of scrapping my case. The rubbermaid was really easy and comes sealed.

Am I right in thinking that humidity needs to stay high?

I think humidity is really important for new seedlings and the first week or two of life. I believe a lot of people only mist water their seedlings. Might be a crazy idea, but maybe you can find an old cpap machine with a little humidifier built in.

A cut pop bottle does the trick for the first week or two, then you're fine in my experience.
I have a small indoor situation and have learned they tend to really outgrow their space quick if you dont plan ahead. Check out my threads.