There's a guy that makes Platens on this forum. If you search the forum for CNC Vacuum Platen his post with other's who've commented should come up.
He sales the sleeve platens for something like $15 or so. You can buy up to four for platens for the same shipping cost.
Also, I wouldn't lay the shirt flat, but pull the sleeve over the platen. Line the sleeve up where you want the imprint. Don't forget to spray a little adhesive/tact first.