Can you tell me whether InstaMorph and/or Thermo-loc are in and of themselves adhesive or would an adhesive be used in addition to these products to hold furniture for engraving?
The InstaMorph stuff is not adhesive by itself, though apparently it can bond to some plastics. It's just plastic that's really easy to melt and form. Think the recommended water temp is something like 150 degrees Fahrenheit. In order for it to hold something, you'd need a mechanical advantage. A really rough surface, pins, pushing it over the lip of a part ect..
I've used it to hold awkward parts, minor indoor plastics repairs, or to quickly add threads to something. You can form it around a bolt, then unscrew it after it cools.
If thermolock is more glue like, that may be what you want. Similar products, perhaps slightly different applications.