A golf ball or billiard ball also works well, depending on the size of the area you're working on. I wish they were like pilot's helmets where they make a wax mold of the head and use it to make a liner that fits like a glove.
15 BMW RnineT
10 Ducati 1098 Streetfighter S - "Sleipnir"
09 Kaw Versys
67 Triumph Bonneville TT Special