under the lean-tos

A lean-to may or may not have walls, or something which passes for walls. Basically, it is any form of shack constructed with two upright posts, across which a horizontal branch or beam of wood is laid. Then something is laid at an angle to the ground to form a roof. Viewed from the front, there is a rectangle, described by the groung, the two verticle posts, and the horizontal beam or pole. Viewed from the side, there is a right triangle, described by the ground, the verticle pole, and whatever passes for a roof.

The roof could be a sheet of plywood, or corrugated iron (often referred to as "tin," making it a "tin-roofed shack"), or simply palm fronds laid close together to shed the rain. In the Philippines, i would guess you have two posts made of tree branches, across which is laid another branch, with palm fronds propped from the ground to the horizontal branch to form the shelter. Hard to say, though, without more context.

Additional information:
They probably didn't have side walls, because a SEAL "could see directly into two of the lean-tos" from afar.
But:
"Two men were busy tying down a tarp over one of the lean-tos as they prepared for the storm."
How do you imagine the latter situation?

OK, to prevent rain from getting there.
It was an abandoned village and the lean-tos could house even eight men. Something like this?:
http://images.google.com/images?q=tbn:2Yg4wZd6bDMZAM:3l337.org/photo2/lean-to%2520shanty.jpg

Ok, that image you had LP, looks like someone's collapsed house--but the basic principle is there. You have an upright support at the front, and some object or collection of objects laid at an angle to form a roof.

The leantos described could have side walls, but if they were facing each other, people could still see into them--there is no wall at the front.

I finally decided on something like "huts with slanted roofs". Once these lean-tos are called huts in the text.
The last problem are the Mark Fives and I went to a military forum with them.
The translation is 99.99 % ready.