Have had to stain a few that soiled in dirt. I use a airbrush for that. Here's how I cut to get the brain out. 3/4" slot and I make a insert to go in. Jig saw or ultrasonic tool is best. I can secure to plaque by screwing into wood. I just use hot glue to lock into position.

Oxyclean is about a pint can for like $6, I'll add half into the couple gallons of water it takes. Simmer skull outside though. Hope this helps.