Is this vacuum degassed BTW? (Would imagine so, voids must kill the ultrasonics?)
Obviously, you need a two-piece (or more) mold to enclose it; or if it's still 1-piece, it's got to be fairly deep and slightly tapered (draft), and leaves a smaller free surface. Molds can be rigid, or silicone rubber -- allowing some undercut and quite complex shapes to be made. Filling sprue should be at the top, with a generous funnel to allow for expansion when vacuuming, or at least pouring when filling. Then cut off the excess and you're good.
Tim