Depending on the embossing height, if high enough I'd use a combination of layered leather bits to get up close to the level needed then dust goop for the final shape. The main difference is that using just stacked leather, you then have to remove leather to get to the final shape, sort of like whittling wood it's a subtractive process. Goop is more like working with clay, you pack it on and then move it around to get your shape, more of an additive process. I haven't done anything practical with either as I'm a crap sculptor but I did play around a little just to see what it's like several years ago.
- Bill