After that I'll make a two part water soluble glaze with weldbond or a good strong white glue, then I'll pour as many glazes on the inside as it takes to get it sounding right to my ears. The trick is not to overglaze and fill up or round off any holes or rough edges to the point of nullifying any of the warmth. Keep it thin and work in layers. Unless of course its kind of a smooth bore, if that's the case I 'll lay on some envirotex which in theory gives it a crisp tight harmonic. I also haven't noticed any change of sound with my synthetic pieces.
Material decides from the get go what the shape is going to be in general.
Wood has more of an innate design already there. I just have to listen and feel for what the wood wants. I add my own touches in the last half of making a wooden piece. Now starting off with synthetics is more like taking a seed of inspiration and cultivating it to get to the point of determining shape and design. Yet every formula I've seen for determining such things always leaves me a little off.