The key is to save as an
object, not a door. (The host door is THE door ... you are just making a panel. I know it is confusing, especially without documentation!)
Make your desired door panel as a slab, flat on the ground, select all elements of it, and save it as an object from the File | GDL Objects menu. (Since you're saving as an object, you can save from either the 2D or 3D window, actually.)
When I've done this I've modeled the slab with the lower left corner on the actual origin. I haven't tested to verify if this is necessary here.
Naturally, save into a project library folder that is loaded by library manager, and then do just as you did and type the name that you saved the slab as into the "Custom Panel Name" field in the door object settings dialog.
Some doors had a bug that prevents custom panels from working ... I can talk about the repair if that is an issue, too. (In 8.1, this bug is in the D1 Entrance 81 door, for example.)
Karl
PS Attached is the only doc about this that I know of ... part of a PDF from GS on storefront objects for some reason. Maybe there's a new doc in 9...didn't search.
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB