case sensor is just meant to hang in the case and get the ambient air.
The CPU sensor can sometimes be fit in the middle of the socket, meaning you gotta take out the cpu, fit the wire between the pins, so that the sensor is in the middle of the socket under the CPU, some wires are too thick though and obviously this isn't an option.
Other best bet is to try and get the sensor next to the CPU die. NOT ON IT though.
Get it next to the die if you can because there's that tiny amount of space between the CPU and the heatsink due to the core being raised.
If yours has a heatspreader on it, then you'd have to simply attach it to the side of the heatspreader or something.
Don't crush the sensor though