There are thermistor cables on the market with tips & attachement cables that are fine enough to slip under the cpu. Some Thermaltake models used to come with such.
https://www.overclockers.com/thermaltake-volcano-9/

So there seems to be some open areas on top of the processor outside of the fan that work well per the red areas in the pic.
Obviously the temp readings there won't be perfect but should be close.

Any ideas on how to safely attach the sensor to the top of the processor?

Thanks!

if you don't mind, you can use some type of thermal glue: https://ro.farnell.com/mg-chemicals/946 ... price=true and place the sensor as near as you can, maybe on the side area of the heat sink that touches the cpu, as close to the 'middle' and as 'near' to the cpu die as possible.

Ordered some double sided heat sink tape from Amazon.

Should fit the bill nicely.