A head cast requires at least two people and the person having the cast made 🙂 And money for the good alginate (plaster heats up very quickly and is not recommend for a full face/head cast.)
I did recently do the tour at Weta Cave (small window into the workshop, omg the CNC machines which are all handbuilt…..) and spoke with well… everyone. 😉 But one artist said what I was doing was fine 🙂 You do need to be able to think in 3D shapes and take your own measurements pretty well. 
I’ve been sculpting since I was a kid. Mainly elfy type creatures and then again a few realistic and a few poupee types. So you can in fact make a sculpture taking all that in to account. Even sculpt it underneath in a different coloured clay.

Were it to be a foam piece I would have definitely waited until I had a cast. To mold and cure foam latex you really do need a mold that holds the final shape. With foam that really means having a backing that will match the final shape it will be adhered to.

As she is slip cast I have a little more freedom as I can glue in foam (urethane either cut from sheets or a low shorw two part urethane- which is incredibly fiddly and requires perfect environmental factors for it to go off with good cell structure).

Before the mold dies completely I’m going to cast her in a semi rigid urethane so I can have a permanent display piece 🙂 I’m currently prepping my Asari molds for the last four or five casts I can really get out of them and will do the same. I have my Samara and Liara tentacles for display but I’d like another set that will never degrade;)

For my next project, the Silurian Madame Vastra I will have to have a head cast. Preferably my own because the scales are so very delicate and fine and blend so closely to the eyes. I would ideally then cast them in foam latex and use a mix of paints and gloss to get that lovely rigid scale look. I’m pretty sure the originals are foam   latex based on how the pieces move.

I do have a back up plan in case I am unable to deal with a head cast (I am very very claustrophobic and do not cope with restricted breathing- this is why you need two helpers to make it go faster and also so there is always someone able to help you breathe/understand what is going on.) 

BTW I have made gelatin, slip latex and silicon appliances. All have they pros and cons. Some are more delicate (encapsulated silicon which I haven’t done) some are easier to apply some look great except where glued (slip cast latex, sigh). It’s fun to try out each kind (highly recommend gelatin for those at home and who can do small partial face casts! The molds do eventually go moldy ditto with the casts but they are cheap and beautifully translucent).

So long wall of text is long 😉 But there are ways around a lot of issues with not having a full range of kit but it does take extra work and practice. And I am most definitely getting a head cast asap. But after my sinuses and glands have de-swollen. I really don’t need them captured forever 😉