I had 3 old broken mouse. Two of them was "cheap Chinese" and one Logitech M235. All of them had something broken.  One was totally dead.

I took out electronics and I soldered out everything what unnecessary. I took scroll and all buttons.  If You soldering mouse like Logitech You have to look out, because it is very delicate. I got rid of one track from the temperature, but I saved it with glue. Then in I soldered 4  wires in place of left and right button and on the end of them wires I soldered the buttons. But before that, I bent the pins.

I dont know yet a real shape, so it will be flexible with long wires. At the end I soldered the battery wires. When I finished it was looking like on the picture bellow.

Next step will be "Looking for best shape".

Edit:

I removed fast switch plug, because it took too much space and soldered two wires.

After many tries ...

... finally I've made basic shape. In the future it will be evolve, but for now is enough.

At the beginning I took "plus" and "minus" form battery holder and put it to my new one.

Then I attached electronics to the bottom of the mouse. I did it in very simple way. I used the 6mm screws. But if You follow You can use even glue. It first prototype so I take what I have. Also in second one I would like to use those usb rechagerable AAA battery, which I found few days ago on Aliexpress.

I've made one button on the printer. But definitely 0,8 nozzle is to thick for such element. But it's look like it's working for now. The clicker I attached with glue.

No it's time to do second one(it's a little different) and put them to the body.

I cut off rest parts from scroll, because they took up too much space. And I glued all part with glue. To mount top and bottom I used a gray tape. This is first prototype so I keep this simple. It's working, idea is good, but for now I know that in Mouse 2.0 I have to change buttons construction(simpler and a little higher), everything should have less parts, battery for accumulator with USB. And definitely I need to use 0,4 nozle in 3d prinnter ...not 0,8 . The bassic shape is good but could be more ergonomic. Mouse 2.0 can also have a rubber or plastic hanlde to hold the hand while lifting.