In the enlarger with 35mm there are usually glassless carriers. These are crap! Single sided glass carriers are the best.
When you focus with a negative in the enlarger using a glassless carrier and get it bitingly sharp by using a grain focussing device. Then go away to get the piece of paper by the time you are ready to start, the residual heat from the lamp will have warped the film and knocked it out of focus. This is one of the facts of life. The warp is always upwards so the single sided glass carrier stops this
If you cannot get a single or double sided carrier,the easiest way around it is to focus the negative, leave for a couple of minutes with the light on whilst you get the paper ready and then re-focus. Once you are happy turn the lamp off and put your paper in the frame and expose normally so the negative doesn't have a chance to cool and warp back to the original shape.
Thank you, you have made an important point.
Warning!! Handling a Hasselblad can be harmful to your financial well being!
Nothing beats a great piece of glass!
I leave the digital work for the urologists and proctologists.