League City, your in a difficult area of Texas, the deep south where humidity levels are oppressive.
A few thrips to Hoouston was enough for me. hehe!
That's what I think your problem is, the ambient or room temperature at the venue. When you put the moist igloo on top, you've got all those factors crating the problem.
Sometimes in the winter I'll come in to a bit of fogging on my front windows. Interior is just warmer enough to meet up with the cold from outside on the windows.
Mikeys suggestion is great, just cover the silica jel packet with some decorative, stones, marbles,jewels. (Great idea Mikey!!)
If you have the time at the venue, don't place the igloo tops on till the last minute, that should allow the vases to acclimate to the venues humidity levels.