Here is some info on how to make your own bathtub floor to integrate with your tarp set up. For Tarp camping or whatever, this is a decent $20 answer to the myog/diy bathtub floor.
GENERIC TUB FLOOR.
Turn a groundsheet into a Tub Floor, by folding the corners out in a certain way.
The reinforced grommets in the corners will interfere with the folding plan, but there should be enough flex in the tarp to make the folds 'near enough'.
About 6 inches (15 cm) in from all 4 sides of the tarp, make a crease line. This will become the line at which the groundsheet ceases to be the 'floor', and becomes the 'wall' of the tub floor. In the 4 corners of the tarp, the crease lines will overlap, making corner squares.
Make diagonal folds in the corner squares, with the fold line coming in from the outermost corner.
Fold up the 'walls' of the Tub floor. The diagonal creases in the corner squares will allow spare material to fold 'bellows' style, into flaps that can point inside or outside of the walls of the Tub floor. Point them outside and use TearAid gear tape to tape a tube in place vertically here. It should be just a little longer than you wall hight. Tape the folds in place and stitch if needed. Tube helps support wall with cord or stake.
3” to 4” high walls works well.
NOTE: If using stakes thru tubes you will want 7” or longer tubular hook stakes that fit thru tube in fold. The tubes from bic pens or even good thick straws work well. Cut them down.