We had to make the witch melt for "The Wizard of Oz". The tricky part is getting the actor in and/or out safely. We used a ladder under the hole at the downstage edge. I bolted a short ladder to the under side of a platform that was built to the exact height for the ladder. The actor steps backward into the hole and a stagehand below the platform grabs the ankles of the actor to safely guide her feet onto each the step of the ladder. The trap door needs to be pretty secure so be sure to use substantial hinges and multiple hefty latches.