What kind of notebook are you talking abot a notebook laptop or a paper notebook in which you write ?

I've had this for ten years. Don't take many notes right enough!

I splurged on a hardcover notebook. I like to write in bed and on-the-go so hardcover is the only option that really works for me.

Assuming you talk about a paper-ringbook-type of thing to write on with a pen, you could buy the ones where the last page is made of cardboard to prevent bending. These sizes also come as books, hardcover thing with blank pages inside, hard to bend.

If we talk about an electronic device, just please dont sit on it ;)

I just don't mind if it gets bent. That's a pretty natural way for a notebook to be.

Use Pritt or a rubber band or something to fix a 3mm A6 plastic or plywood sheet to the back of the notebook?

Or, you can buy A6 clipboards.

I eventually just gave up and started buying expensive notebooks. The cheap ones always fell apart before I finished them, so at least this way I get to the end!

It kind of depends on application?

I carry a small (4”?) “field” note book for jotting things down. They fit breast pockets easily, if that gives you an idea of size.

If it’s just personal notes, or something, those work fine, they also make medium size (“stenno” pads) that are about half the size of an A6.

Composite notebooks are great too, though they’re not quite A6 and finding them unlined is a chore (if your note taking frequently includes sketches).

Most not-dirt-cheap stenos will either simply tolerate bending or have enough on it that it doesn’t bend too easily.

Put it in the chest pocket of your jacket, that way it stays flat.

i'd look for a small folio type case. something durable, maybe leather-ish.