A few days after building the wall, when all the structural masonry had some time to cure, we started on the rest of the construction. My plan was to use two pressure-treated 2 x 10 timbers to build the back wall, supported by three 4 x 4 posts that would extend up to support a trellis. I dug three post holes, 20" deep, for the three 4 x 4 x 10' trellis posts; one behind each end of the wall, and one in the middle of the bed.
Four 3/8" x 6" galvanized lag screws were used on each side to attach through the posts into the masonry, sandwiching the 2 x 10's. I countersunk the heads of the lag screws 1-1/2" into the posts with a 1-1/8" forstner bit so that they'd be hidden and also so that the 6" screws would penetrate far enough into the masonry wall. We held everything in place while I used a long wood bit to drill through the posts and boards, and then a 1/2" bit in the hammer drill to reach through and start holes in the concrete before removing the wood temporarily. I used a hammer drill to install eight lag shield anchors into 2-1/2" x 5/8" holes in the back before fastening everything with the lag bolts. The post holes were backfilled with packed soil, instead of pouring a concrete footing. Everything feels extremely solid, but shouldn't be a nightmare to take apart if we need to for some reason.
We immediately went and got a yard of compost mix and filled the planter up. I'm not sure that was a good idea, but I guess, worst case, we shovel it all out some time and adjust the structure. The main thing I'm worried about is the wood back. I think there should have been a plastic sheet liner installed between it and the soil. It would help the wood last longer (even p.t. eventually rots), it would keep nasty preservatives from leaching into the planter's soil, and it would keep the soil at the back of the planter from drying out. The drying issue may be a problem against the concrete, too.