[ckissick]

I'm not quite sure what the walls look like; a pic would be nice. Anyway, I would just make forms for the risers. It's not that hard.

You can make a form for each riser by using 2 overlapping lengths of plywood that are the length of the riser. If you butt up one side against a rock, you can trace the shape of the rock onto the plywood by moving something like a ruler along the rock, while drawing the same shape on the plywood on the other side of the ruler. Cut along the line.

When both ends are cut, adjust them so they are the exact length they need to be, then attach the two pieces of plywood to each other. You'll probably want the face of the riser to not have a step in it, so the two pieces can act as a template for the final single piece of plywood form. Attach the forms with metal stakes, but make sure you'll be able to remove the stakes after the concrete sets. The forms will want to bow out in the middle, so have a stake in the middle.

Making forms may be a little time consuming, but it will make the job go much smoother in the long run.