Step 2: Adding the armature
Safety tip: Make sure to wear gloves during this step; it's all to easy to stab or slice your finger open.
Using a pair of tin snips cut, or slightly notch, the sides of the stud, then bend them to shape. I started by screwing down a section of a stud about a foot from the end to the top of the the plywood. Then I folded the ends together, overlapping the sides, and fastened with sheet metal screws. Working my up I notched, bent, and fastened the stud every so often to form a curved shape. I made three "arms" this way jutting out from the plywood at different angles and heights.
Also shown in this step is building up the outside of the fake sewer opening with a length of vinyl trim (this was some sort of flashing also found at a home improvement store). This particular piece of vinyl has two ridges running down its length which made it easier to center over the short 1x2 boards. I painted both sides of the vinyl, wrapped it around, cut it to length, then screwed it into place (through the vinyl into the 1x2s).
The final picture shows a test placement over the top of our manhole. The boards rest solidly on sewer and the vinyl extends around the outside, down to the ground.