Does anyone know approximate dimensions of lean tos at Roaring Brook? Wondering about putting a sceen tent in to keep insects away.

You can't put any tents in the lean too. I recommend therma cells with screen material across the front of the lean to

The entrance is about 10 feet high and 20 feet wide. The roof slopes down towards the back

Considering the size of the lean tos. A small tent secured to the lean-to should work as stated by David H.

You may be able to pitch a small tent in a lean-to, or at least secure the tent with string to the lean-to, I've done that to get out of the flies.

A bivy sack with a bug screen will work well for the space. Don't forget your mattress pad!

A screen is a good idea but your typical screen tents won't work because of the slant.