So far we have tried using a hard ball syrup as a glue, but it is challenging to evenly coat the popcorn and get consistent results. We have tried drizzling the hot syrup over warmed popcorn and then mixing by hand in a large bowl but the syrup just does not spread out enough. We'd love to be able to replicate the process they use to make Popcorn cakes where they seem to be breaking down the starch and then solidifying it.  Any and all suggestions welcome! 
We ended up using straight corn syrup heated to soft ball stage. Two critical factors were (1) use the right proportions and (2) do the mixing/coating inside a big wok to keep it from cooling before adding to the mold.  Getting the right proportions takes trial and error. Too little corn syrup will not hold the popcorn together well and too much will make the mixture too sticky to press into the mold. We ended up using about 1/4 cup per batch of air-popped corn. The biggest wok we could find was 18" but even bigger would have probably have been even better. 