I don't have tips, but I can share my experience. In Fukuoka where the stuff is very popular, I toured a factory which offered the experience of making it yourself. They supplied frozen roe and the seasonings. You mixed it yourself.
They recommended keeping it refrigerated for a couple days for the seasonings to penetrate. I was driving around the country at the time, so I didn't have the luxury. I kept it for a week at whatever temp it was in the car or bag, and it actually turned out to be the best mentaiko I've tasted.
