[Therion] Height numbers in square boxes

Hello everyone - A common convention around Missouri cave mapping is to indicate the height change of a floor step by enclosing the value in a square box. This is not the same as ceiling height which is enclosed in a circle or oval. I have been using the object "point height" for this. It works except there is nothing around the value. It just hangs out in space, so to speak. I used the "-d" option to dump all the MetaPost and TeX code. I find no references in any of the files to objects of type height or passage-height, which implies that these two objects are not generated by MetaPost. Is it possible to redefine the "point height" symbol so it draws a square box around the value? Thanks! -- Bill Gee