Simple fix.
Either get some wooden dowels or something like this from MCmaster....
Spacers and Standoffs
Type Female Unthreaded Round
Female Unthreaded Round Type With Shoulder
Take you core, drill 2 holes in it, place these inside, put the base in the mold, cassette, whatever, put some super glue on the bottom of the dowel, line of the core in the right spot, let the dowels become glued in place, pull the core off, and you are set.
Just make a little hole in the glass when you lay up the skis and the core will not shift at all.
This is a solution many small builders use who do not have access to professional cassettes.
Eventually I am going to get full on cassette that are laser cut for the base material and have rails for the core, but those are a bit out of my price range right now.