I've used the old style Virga tarptent which is floorless. I've sat in that thing during some horrendous rains and high winds. Some spray would get in but I learned to pull down on the winward side and lower the pole for a more weather resisitant shelter. You can cut tyvek or plastic to the size you want for a ground cloth (floor) and then easy to replace when the time comes.

Give it a try. <img src="/forums/images/graemlins/laugh.gif" alt="" />