Mike

Ok Darren, now that we are on the same sheet of music I have a suggestion. Both Shopnotes and Woodsmith have plans for very nice plywood boxes with ideas on dressing them up for different looks, complete with the cushion on top.
There is no way you can freehand your finger joints accurate enough unless you use a template or jig to guide you. The easiest way I can think of is to purchase a box joint guide plate for a dovetail jig. You can screw this to some sacrificial wood and clamp it to the end of a bench, with the "comb" shape facing up. Use a guide bushing with your bit and you should be able to get nice results. You will want to cut both pieces of wood at the same time to ensure a good fit. I would lay one sheet down, apply some double sided carpet tape to it and set the other sheet on top with a 3/4" offset and then clamp it down. Another way would be to build a jig to attach to the bottom of your router. If you look at the Oak Park site you will see Bob & Ricks box joint jig. Normally this type of jig is mounted to a table and you stand your wood on end and move over it. That's not practical for your application, but you can attach a jig like this to your router and using the end of a bench for support should get great results. If you need help building a jig let me know.

__________________
Mike