I used to use the Double Heat Sink baseCoolingRate Fix by Moddingisthegame found here, but it has not been updated for years at this point. So I ended up making my own up-to-date version from scratch. I hadn't really intended to upload it, but I noticed the other mod being mentioned here and there, so maybe this will be of interest to someone.

I've doubled the base cooling rate of mechs with internal double heat sinks, rounding down to a single digit and without touching the total
heat capacity, in the interest of balance.

This mod should also avoid the issue with lowered maintenance costs, and fix the CPLT-C1B that the game had wrongly listed as using singles.

It requires:
-Heroes of the Inner Sphere
-Legend of the Kestral Lancers
-Call to Arms
-Rise of Rasalhague
-Dragon's Gambit

To install it's the usual procedure:
-Unzip into your mods folder. (should be ...MW5Mercs\MW5Mercs\Mods)
-Launch the game and enable DHSFixUpdated.
-Relaunch the game and it should work!

It's a very simple mod, mostly just takes a bit of time to do, but my understanding of the editor is pretty limited. I can only say I haven't noticed any issues so far. Let me know if you run into any or if I've missed specific mechs.

1.3: Updated for Dragon's Gambit
1.2: Added the ZEU-5T
1.1: First upload