I don't know there is a proper way to stretch rackets. If I were compressing a racket a little bit I may be inclined to stretch it a little bit to getting the right end result but you are talking about 1cm. That is not a little bit that almost 1/2".
I used to string on a old stringer that had a rod that was placed at 3 & 9 while stringing to hold the sides in while the mains were strung. Maybe you could use scrap string to do the same thing. The first thing I would do is find out how and when it is compressing and work from there.
All you higher end machine move the posts from the bottom (with micro adjustments at the top) when mounting rackets. You turn knobs connected to a threaded rod the move the supports in and out. You can try to do the same thing. Mount the frame and pull the mounting support from the bottom. This will help to pre-load the support by removing any lateral movement while you tighten the post.
When you're dead you don't know you're dead. It's the same way when you're stupid.