favorite field shelter is a tent that I hope to have with me. I like something that can keep uninvited things from sharing my sleeping quarters. If I did not have the tent however I would say a lean to because it is pretty simple to construct.
if i have a tent i use that. if you have a good tarp, the possibilities are endless. but if i lost my ruck somehow, a lean to with a reflector fire is pretty comfortable. but you know ol' boris. i aways try to have something with me. even a trash bag can b use to help construct shelter. why i carry a big roll in the van