So you are suggesting to put a heat sink on top of Rasp with good thermal paste. Correct?
Thermal paste by itself would not be enough to keep the heat sink in place, unfortunately. If you have epoxy glue available, mix a tiny amount of it with thermal paste and use it to glue the heat sink in place (I would use 2-3 parts of thermal paste for 1 of epoxy, so that it would be relatively easy to remove if you wish. Or the same ratio of sealant and thermal paste. You won't need a lot of strength in the glue, and many other glue mixes will probably work equally well. All you need is a bit of adhesion on top of the thermal paste. Or buy a proven good thermal pad from a reputable source (e.g. Sparkfun in the USA).
Or, even better, buy one of those finned aluminum cases, where the case itself is the heat sink and retained in place with screws, and then you can use thermal paste. Just be aware that most are not that well designed, see my review on the one I bought from Amazon
https://smile.amazon.com/gp/custome...=cm_cr_getr_d_rvw_ttl?ie=UTF8&ASIN=B07TZQHXZ6 I use that case purely as a passive case, since mi RPi is not fully loaded, so I disconnected the fans. Can use the fans at 5V or 3.3V (the latter quieter and still plenty good cooling). Overall, I'm really happy with that case now
EDIT: just in case, and since I mentioned it before, I also used this to glue heat sinks for a variety of uses
https://www.ebay.com/itm/174734313048. Please note I'm not suggesting to use that specific seller, just the first one I found when searching. It works well and it's cheap, strong enough even in case of vibrations, and decent heat transfer (in any case, you use just a very, very thin layer, ideally you want metal to metal contact). The only downside is that once you open the tube (and it's a tiny tube, just in case it's not obvious from the picture), it starts hardening and within a month or two is mostly all gone.