Thanks all ...

Think I'll try using several of the techniques described along with one I came up with on my own.

I'm going to run a dado down the center of a plank and then slice off narrow pieces to use as the cleats to hold the forms. That way, I can position the forms by pushing wedges between them and the box beam into the "tunnel" created by the dado.

I'll use the string techniques described in the responses to check the alignment and then hot glue the forms prior to driving in screws - probably also predrill screw holes in the forms before I slide them onto the box beam just to make it even less likely that I'll disturb the alignment.

EdP