What you will need is of course PU insulating foam. They are usually well stock in DIY stores, comes in a can slightly bigger than a spray can, commonly use to fill bellowing gaps and cracks, hence 'insulation' aptly named, grab a few cans to cut those energy bills! You will need maybe one can, at most two.
Next Sellotape or Scotch tape, this does magic to create a semi rigid shell. You will need a roll of two. If you don't know what I mean, please see what this guy (Ryo Sehata) has done with Sellotape.
Extras, paper mache mix or car filler (polyester repair paste), a sharp pen knife, steady hands, experienced handling of whip cream, latex gloves and patient! This project will take 2 days, party because the foam take a bit of time to expand, take shape and settle.
But just to show you the front view of the almost final result covered in of paper mache. The rest is up to your creativity at decorating it! Had Super Mario's mushroom in mind, never got to doing it.
First invite unsuspecting test subject, grab a suitably thick towel, wrap it round like a head scarves, secure with a knot. This ensures a comfortable fit and allow for some shrinkage due to the expanding foam. Better still, it allows padded inlays if you so desire that luxury.
Next, place sheet(s) of cling film over the head, loosely estimate the coverage, excess can be trimmed later on.
Now, by layering the scotch tape over the cling film, which can be a bit tricky but persist! It gets easier as the layers take shape and foam a semi-rigid shell.
See the steps of progression below.
Warning! PU foam is extremely hard to remove once it is cured, so avoid staining on ANYTHING. Grab that part dungarees and work those farm hands!
The next bit is also rather tricky, that is to apply the foam onto the shell. For foam virgins, it oozes like shaving cream, and handles like whip cream.
Warning! Do not be too enthusiastic with the foam application! As mentioned, too much is a bad thing, the foam will simply 'fall off'
Go easy on the first application, when it is dry to touch then can you be more liberal with the foam, as it sticks and 'hangs' better.
If you prefer the regular look, shave the foam with a pen knife, the voids can be repaired with further application of foam.
Warning! It might be daft to say this BUT this is not a crash helmet and it is probably not safe to wear it doing monster truck demonstration.
Lastly, special thanks to Gaby the helmet model!