Looking for a solution as well! I like the cement mixer idea. I was just going to dump a bag into a large plastic tote box, toss in a bit of water and innoculant and mix with a shovel.

I’ve mixed in a drill box successfully before, but the planter boxes are a little trickier.

Another idea. A neighbor uses a paint stirrer on a drill to mix in graphite. Wouldn’t get it coated as well as cement mixer, but might be better than just tossing it on top.