From time to time, you come across things that make you think, "This has to change."
I had this experience with sanding large surfaces. Small pieces of sandpaper, folded or not, to sand more easily and powerfully. The result was that it wasn't always straight, leading to annoyance.
Until the point where change happens.
Take a piece of sandpaper, either flat or a sanding belt.
Some wood, in this case, I used MDF.
Clamps
Wood glue
Knife for trimming, if necessary.
Cover the entire surface with wood glue.
Secure the entire assembly with clamps and let it dry for a day.
Then trim the paper.
Perfect.
A large piece of flat sandpaper.
Sturdy and won't fold during sanding.
I immediately made two, with different grits.
ADDITION TO THIS TIP
I received a tip to use waterproof plywood instead of MDF.
And for the following reason:
For example, if the sandpaper gets clogged with plaster, you can rinse the sanding block under the tap if you use waterproof plywood.
Let it dry and it's ready for next time.
(Thank you, Martin, for this additional TIP).