Here's a quick version of a basic door
unit I've had some success with, and a rough drawing that should hopefully make it fairly
clear how it's made. The drawing is roughly based on a 32" x 80" door, adjust the measurements as needed if your door is a different size. The process is as follows:
-Start with a pre-hung door from a home center. You can do this without the pre-hung frame if you already have a door you want to use, but buying pre-hung saves you installing hinges and a doorjamb.
-Add a 2x4 or 2x6 frame on top of and on the sides of the pre-hung door, give yourself an extra 1/2" or so on the bottom (more if you have a particularly uneven
stage) to allow for any flex in the frame when the door moves around
-Add a piece of 1/8" x 2" steel or aluminum
flat stock to connect the bottom of your 2x4 or 2x6 frame together
-Add 6" x 6" steel
flat L-brackets to the top corners of the frame to help keep it from racking (if you want longer you can get bigger L brackets or old framing squares and drill them out). You can use plywood triangles instead as long as they are on the side the door doesn't open towards, but that's a
bit less visually pleasing
-Build
caster plates from a 2' 2x4 (longer if you're going to use a taller than standard door or if your actors are going to be rough with it), a 2' x 1' piece of plywood cut into a trapezoid-ish shape (see the drawing) and a couple of 2x4s cut at angles to hold them together
-Use total-lock casters if you can (these lock both the swivel and the wheel). I like the 4" blue rubber ones from Rose Brand, or the 3" red ones from Home Depot.
-Attach your
caster plates to the sides of your frame so that the bottom of the wheels gives you at least 1/4" between the floor and the
flat stock on your frame (allow for more room if you have an uneven
stage)
Good luck, and have a happy new year!