Thanks for all the compliments everyone. I ended up using the coco liners, but still had to plant them with the liner in the box first. It wasn't too bad, but I am bummed because now that the boxes are already installed, the lip of the sill is over the box so I won't ever be able to fit pvc liners in there. Oh well...live and learn. Here are some updated photos of the boxes.
They look great, but I find geraniums to be kind of a lot of work (deadheading). Any suggestions for next year? Something easy that you don't have to deadhead.