Before you go down this path how about first ID’ing the root cause of the heat buildup. Install this great App [https://www.tunabellysoftware.com/tgpro/|TG Pro] given the age of your system you should also review your batteries health as your system will work harder (more heat) trying to charge the battery. Again here’s a great App [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] these two apps are my go to. I’ve used them for years on my systems and fully recommend them.
Post a snapshot of both when you are encountering thermal issues, so we can see what’s happening. For TG Pro the apps main window showing all of the sensors (don’t forget the ones which are below as you’ll need a second capture for them. For CoconutBattery while the charger is plugged in and the MagSafe LED is Amber, as well as the battery section expanded.
***FYI*** - I’ve never needed to improve the heat dissipation into the bottom cover in this series or even the 15” models (unibody’s) I’ve often needed to replace the heatsink as the heat transfer tube coolant has leaked out and/or the thermal paste was poor quality or incorrectly applied.
Once we see the data we should be able to aim you on what’s needed