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I'm by no means trained or anything so take what I say with a pinch of salt but a very quick shelter I use everytime I go out fits the bill of what you are looking for.
Anyone that knows their stuff will see right through it but for a 30 minute shelter that will stop the average joe seeing you this is the way to go.
-Find a tree that has fallen over
-go to the hole left by roots
-chuck big sticks and other deadfall across the hole
-cover in leaves and twigs
a basic one will take no more than an hour, be relatively hidden and pretty waterproof