Here's what I would do to get the perfect template. Use the plastic to get a mould of your head. Then set that aside once it hah hardened and turned back to white (it is clear when soft). Use the saran wrap and tape method to make your templat and then once that is the perfect size and shape, set it on top of the plastic mould and draw around it. That way you will know where to cut the plastic mould. It can be hard to do it with just the plastic because it turns back to white opaque as it hardens and then you can't see where to draw the lines. If the factories accept hard plastic opaque templates I would use them absolutely!

The stuff you want is called polymorph: http://www.youtube.com/watch?v=sfcBWnOqr6s

It's amazing stuff. You can even make keyrings, toys, jewelry, spare parts for things you broke etc. And it's cheap. We're talking about $10-15 for 500 grams.

Make a big soft smooth ball of it and then roll it out on a flat surface with a rolling pin. You will have to keep putting it back into a basin of hot water to keep it soft as you roll it some more to get it even. Once you have a sheet big enough to cover yor head and about 5mm-10mm thick put it back into the water to soften (it turns clear when ready). Then pull it over your scalp making sure everywhere is covered and it's a smooth tight contact. Let it cool (it takes only 5 to 10 minutes or so) and you will have a hard plastic mould. Use the normal saran wrap method to get the right template shape and then use that to guide you where to cut the mould.

I have done it before and made the perfect mould of the top of my head and it was really solid. But I thought for some reason the factory needed a flexible and transparent template to work with.