MD007, Went thru your cycle when started. Went with water helmet (like tow water park ones) this happened to have open ear area and like most rigids doesn't offer too much around the kneck back of head area. Never hit on head by kit wearing this, but water impact on repeat and into ears when crashing (v.frequent at what feels like hi speed!!) became too much. Often had to stop/give up when water logged and punch drunk due to helmet head first into water.

Moved to kit as per picture. Although not padded the sun hat with kneck shade has proven best from kit impacts. I recall 3 hits in zone below padded protection area once with scrum cap on, twice with sun hat. The hat's rim and shade material (same stuff quiet stiff) helped the most..
The way you leave the board when foiling is key. I 95%+ go over front or back still. Once or twice very gentle drops over side onto foil - a wetsuit helps here. Probably foot straps change this a lot. I always have had one on the front but rarely at the back. Never fight hard to stay balanced - until U pick up that alignment feel (like a continuation of the mast!!) - if you have access to a short short mast this is less of an issue as can bounce/balance using the board hitting the water.
Cheers
AP

Edit - in my avatar old old shot wearing an old gath helmet - that blue bit - I do recall it having a bit of a padded rim around the base at the back but long gone b4 foiling..
There is another whole subject on tips tricks and equipment to help.. - if u have native skill U R ahead of the 8-ball already