I have done the research on this couple of years ago.
The scanner idea doesn't work because for negatives you need light coming from BEHIND the film slide, but the scanner is designed to scan paper. So it doesn't scan the negatives very well. I tried to be clever and placed my phone at full brightness behind the negative, but to my surprise that makes the scans black and white! This is because the light you see coming from the scanner slide is not actually white, it's flickering quickly between R/G/B, and the sensor can only detect intensity of light, it's a B&W sensor. So the way they scan color is by quickly switching between R/G/B lights and combining the three B&W values it scanned. So without the scanner controlling the light source, it will not be able to give you a color image.
Because of this reason, the scanners designed to scan transparent material are fundamentally different. Your best bet is giving your negatives to a professional scanning service that has the scanner. This is going to be hard to find, but is worth is because it will only get even harder in future.
The only actual DIY alternative is photographing the negatives with a high quality camera, while they are being backlit from behind. But this is harder than it sounds because photographing a flat surface close to the camera without any DOF blur around the edges is very difficult unless you have a very good and expensive macro lens.
