I just did a quick search around the Internet as I could have swore I saw a kit for the 550 before. But I came up empty.
I would give a call to CDI precision and ask them if they have any plans to make a bottom metal kit for the 550 in the future. They make ones for damn near every common bolt action except the 550 and they're always coming out with new ones. They even make kits for a M98 and model 70 long and short action. Enough phone calls from interested gun nutz might make it a reality.