Taking a copy - what's called a 'squeeze' - goes back a long way. It's a particularly valuable technique for lighter, ornamental carving, the sort of thing you might find in furniture or four-poster beds. You've got to carve the design but you can't take the original carving with you - but you may be able to take a copy!
I'm using a modern material, 'Plastilene'. It needs to be warmed up a bit to make it malleable. I mention that I put mine in the microwave for a few seconds – be careful! Just a few seconds, then test it. You want it soft-but-still-firm. Wrap it in metal kitchen foil to keep it warm until you get to the job.