I feel I need to dive in here. I think using the right tool for the right job applies here. I think trying to sculpt wth milliput would have disastrous results, I find it difficult to work with. Milliput and epoxy fillers etc are for repairing and filling gaps, whilst you can make basic models with it (scenery etc), I think trying to make a detailed maquette out of it is asking for trouble. What you need is Super Sculpey Firm. It's formulated for this very purpose it won't dry out and you can just concentrate on being creative and not struggle trying to manipulate it. You can then bake it in the oven mold it or paint it, its also non toxic and completley food safe so you don't need a different oven. I think previous advice may work but it's long winded and just plain hard work imao.