While I'm not positive about current availability, I think Seal at least used to make big metal platens with a handle on the back for putting weight on a freshly mounted print as it was cooling. If you can't find something like this commercially made, it would be pretty easy to have one made. Just go to a local machine shop, have them weld a handle to the back of a piece of 1/4" steel at the size of your liking, and voila - you're in business.