Force sensor on uneven shape
Use a strain gage.

Super-glue a full wheatstone bridge onto the structure.  Amplify it by 100 to 1000x and your good to go.

Calibration is easy with a precision shunt resistor (to get strain).  Then use some maths to estimate load.

It's a bit pricey for quality gages, and you will still need to acquire the voltages.  But once it's in place, it will last a long time.

If you design it as a in-series module, then you could directly calibrate it with dead-weights or a press, and it's even easier (I think that it's still a good idea to have a shunt resistor so you can detect if the gages are drifting or damaged).

Vishay has some good app notes on the subject.