If you want the final print to be really sharp, you'll need a vacuum frame...
If you're lucky, you can find as a part of an old repro maschine (I an from Denmark, so I don't really know the right english word for it).
I have been offered them for free more than once... They are "useless" in our days, and the top of them is a vacuum frame... which you can unscrew and use.
something like the picture attached (doesn't look exactely like mine..)