Very nicely done; I made a boxjoint jig, and will use that on the corners, which should look really cool, and be super strong. I'd think pine would need to be fairly thick, 5/8" or better to avoid risk of splitting. I might use 3/8 to 1/2" plywood (more stable w/ varied humidity and strong grain both directions), or glued up hardwood scraps. Where did you get that rope? Great idea that avoids LIGHT that other slot style handles allow to leak in.
Mark
The rope is just a plain hemp rope. Should be available from any hardware store. I got a router last year and have been trying different projects with it (just finished trimming a newly installed window with curly fiddleback maple). Dovetails are my next challenge to tackle.