As a manufacturer of Porsche Racing Windshields, I offer the following advice. Stop drill the crack using a #40 bit. Gently de-burr the edges of the crack. Sand the crack starting with 600 wet paper. If need be, fill the crack with a sliver of material leaving a little oh it higher than the canopy mold line. You make this sliver with a wood plane and a fresh sheet of material from the hardware store.

Glue the canopy using Weld-On #4 medium speed glue. After it hardens, preferably the next day, sand the excess material flush and polish with mirror-glaze. (Sand paper through to 2000 grit is available from 3m.)

Email me for more details if you need to .... jscorrea@ncia.com

Note from rec.aviaton.homebuilt (April 19,2003)

It's tricky, but for Plexiglas you can dissolve some small pieces of Plexiglass in Acetone. After stop drilling the crack, use a needle to pick up a drop of teh disolved Plexiglas and touch the end of the crack "inside the stop drilled hole." The thin mixture will wick up the crack and seal int, at least for several inches.

This is one of the "ticklish" operations where a misplaced drop of the Acetone mix can ruin the canopy. If a drop gets on the plastic, don't wipe it off, but rather let it dry and polish it out.