I've used this https://www.amazon.com/Conductive-Silicone-Adhesive-Compound-Heatsink/dp/B07TSH6MNT/ref=psdc_3012290011_t1_B07HCDRL7W with good results on L3+ heatsinks that have fallen off. It worked well, and the heat sinks can be easily removed if you need to reposition.

Good Luck!