Well, here are the ones I made. I didn't try a practice one, went ahead and did the left one first, and then the right. They seal pretty good. The headphones that came with them are OK, nothing phenomenal. I haven't tried them yet under the helmet though....
I did have extra material left over from each ear as I formed them; they do give you enough in each container to mold them. It's not sticky, the two components when kneaded together is sort of like a silly putty.