I drilled the two holes, one for foam and the other for the air to escape.
The holes are the exact same size as the straw you get with the foam.
From the outside of the form I counter bored the two holes out about one inch in, one size under 1/4" to match two dowel plugs that I cut with tapered ends.
When the foam starts to exit the holes, drive in the plugs, cut them flush and put the mold into a wood working vise.