Expand a small hammock tarp with a poncho.

 

A little bit exposed

You can get a fairly cheap hammock with a tarp that is comfortable and suitable for withstanding mild rain. In stormy horizontal rain, you will obviously get wet. But you can use your poncho to make one end waterproof. You can use the snap fasteners to seal up one end of the poncho. Use cord and tent pegs to secure the other end. 

The poncho keeps one end dry

If you have 2 ponchos you can cover both ends. If your poncho is big enough, you won't even need a tarp. Connect 2 identical ponchos with the snap fasteners. With British army ponchos that would give you a 9x6.7 foot sheet with grommets for person lines. That is slightly small though.

If you have 1 tarp and one poncho, you can use a garbage bag on the other end, perhaps cutting 1 corner off the bag to thread the webbing strap through.

You could probably use 2 garbage bags with no poncho, but I always bring a poncho.