Google the phrase "solo bug tent".

Bug tents are often combined with a tarp to create something akin to a full tent experience. The bug tent can be put up alone when bugs are, well, bugging you, or you can put up just the tarp. Some bug tents are designed to work (to clip into) certain tarps.

My personal experience with a couple of bug tents is that they offer very little room inside. Typically you put a lot of your stuff under the tarp but outside of the bug netting. Overall I found this approach to be a bit of a PITA, but it does offer some advantages.

And in answer to the initial question --- typically they do offer some sort of bathtub like floor, though check to make sure how much they promise in terms of being water resistant or 'proof'.
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Brian Lewis
http://postholer.com/brianle