I use the following:
It consists of 2 velcro strips, one stuck to the inside of the helmet, and the matching strip fastens to the first strip. The strip that I've lifted with my hand in the above picture has 3 strips of window-sealing foam stuck to it (the foam strips have an adhesive backing). This means that when my custom helmet pads wear out, i.e. get flat and less cushioned, I can easily replace them. They're also washable.
I used the above approach, because I found that when wearing the
GoPro video camera on the helmet mount, the existing helmet pads weren't soft enough, and so my scalp was hurting, so I used the above solution to increase the padding.