The tried & true way to do this is to use commercial Pith.

Soak the Pith in water.

Make a slit or opening in the Pith that will accomadate the specimen.

Place the specimen into the Pith.

Squeeze the Pith with your fingers to secure the specimen.

Make a transverse slice with a razor blade to provide a uniform flat surface.

Slice sections rapidly as described earlier & float them in a Petri dish of water. Treat these like free-hand sections.

If commercial Pith is unavailable, find a local plant that has a healthy Pith and use it. If the Pith is still alive, cellular debris  may contaminate the sections.