Buy some door skin. Cheap plywood an eighth inch thick. Rip it up 3" rips. Bring the rips along with a hot glue gun to the job site. Bring a block plane too. And a tin snip. Scribe each piece to the wall. Then glue a lap joint in the corner. Then a piece or 2 diagonally. Now you have a template to duplicate.
Cut each mitre big by an eighth. Then set up a router with a straight edge to clean up the joint. Do the biscuits or dominoes. You can glue on clamp blocks to bring the joint tight.
A ledger board fastened to the wall either temporary or permanent will be easier to make the template. Or 2 more hands...
I find stainless to have enough glare that tiny accurate measurements are difficult late in the day. I've had a 6" imperial and the metric version in my pocket every work day since June 2013. I often use the metric one to transfer marks without ever reading the scale. But the primary use for the metric one is setup work using my Domino. I called Woodpeckers just a few days after receiving my Imperial(inch)ruler and they promptly shipped me the metric version. They make extras of the " One time tools " but only limited numbers.