So sorry for responding late....again!
Been really busy these last few weeks.
Thanks Si for posting and I guess it wont be a tutorial w/o a little bit of text - but glad some of you took the time & effort to do this as I'm sure the pics are self-explanatory.
Making the inner skeletal structure could be of any material - card, plywood, styrene, etc. You can even use corrugated card if you want as it wont be seen outside anyway.
Start off with making the base - the one that looks like the top profile of the Millenium Falcon
As a basis, you can use your DP, those Riddell mini buckets and even your 1/6 scale hasbro figures (just multiply by six).
The base is important as it determines the flare of your earcaps as well as the front & rear of your helmet.
Do take note that if your doubtful of your measurements - just bring a ruler in front of a mirror & check it againts your own head.
For starters, just position the earcaps in the middle of the front & rear.
Next would be the center profile. Place a card on top of your base (looks like an inverted letter "T") and mark the front & rear. From the middle, draw a vertical line with a 10-11 degree angle going to the rear. This vertical line could be somewhere between 6"-6.5". At the end of that line, draw a perpendicular line (90 degress) having a total lenght of 9"-9.5". This line would be your "brow" line. At both ends of this line, connect to both front & rear of the base.
Above your brow line would be the dome profile - for starters, make the height of the dome 4"-4.25".
Glue the center profile in the middle of the base - look at it from the front & it looks like an inverted letter "T".
Next, draw an oblong shape (measure your head) & cut it in the middle. Glue each half-oblong on the left & right of the side profile following your brow line. To support it, just place a card under it. Preferably, place the support card in the middle extending up to the earcaps on the base. Now you'll see the extent of your side flare.
Next, above these half-oblongs - make the front profile of your dome. Make a half-circle, cut it in the middle & glue to both right & left portion of the dome center profile. Look at it from above and it looks like the letter "X" or cross.
Now you'll see the shape of your bucket - structurally.
Sheeting - choose a thin sheet of smooth card (you can use an illustration board - black on one side, white on the other).
Envelope the front to rear by gluing the sheet to the base and the brow line. Then cut the excess sheet.
After sheeting, get a thicker card & cut a long strip of 1/2" & glue this strip below the brow line - or the top most of your sheeting - front to back.
On that same thickness of card, draw your mandible design & glue it in front.
Cut the sheeting of both cheeks & built it up following the fett bucket design. Bondo this to shape.
Place crushed newspapers or foam pieces on the dome & bondo to shape following your card profile.
Then your bucket is done - you can make your earcaps & rear keyholes separately if you want.
Well, I hope you kinda understood what I wrote above
I'll answer questions as they come.
Molding: this is still tricky as I have Fiberglass workers working for me - so I just give the card bucket for them to mold.
Basically, make a flange in the middle of your helmet separating the front & rear. Mold the front, after done then mold the back.
There are also other (less expensive) mediums like plaster of paris or maybe cheaper resin. Paper mache (?) could also be a less expensive medium.
I'm not that familiar with other molding materials/chemicals you can find in your area, so you may ask around your local hardware for what could be used.
I've heard of using clay for molding & baking it to make it hard.
Well, I hope that the above tutorial helps a bit!