This is a remix of two wonderful models:

Ultimate Helmet holder by Flobi

Low-poly skull by Peter Farell

where you can use the low-poly skull instead of the original ball that goes on top of the helmet holder main body.  

Printing instructions:

Skull:

• 2 perimeters
• 10% infill, adaptive cubic recommended
• organic supports on build plate only (tune overhang treshold according to what your printer and filament can accomplish - I used 35degrees with PLA) 

Helmet holder:

• Use printing instructions from the original model here
• EDIT: Since the original model was deleted (not sure why - it was a great model): just add an extra wall or two for the main part to make it bit stronger, otherwise you can go pretty much with default settings. No supports needed. 15% infill. Print in PLA, unless you want to mount it outside.
• EDIT: Creator of the original model for the holder asked me to remove the model files as his licensing conditions allow only remixes and those files were just copies of his original files. You can download the files from here: https://makerworld.com/en/models/1222417-ultimate-helmet-holder-v3#profileId-1239501