If you are looking for an easy way out just make a raised standing frame of treated 2x4's with a cross braces in the center large enough to accomodate a small rigid plastic kiddie pool. Puncture the bottom of the pool for drainage and cut out a section (above the level of the filler). Fill the bottom with pea gravel. This way you can wash the gravel all the way to the bottom of the pool where the liquid waste will fall through the frame and with the standing frame you can wash underneath it. Rain will not stay in the pool and the doggies can do their thing in clean gravel.