It won't be pretty but if you can access the CPU from the top or bottom, you can strap a desktop heatsink down with zipties running both ways around the laptop.
In my case, the weight of the fan connected to a 12v motorcycle battery was enough to make it not throttle at indefinite 100% load, I didn’t have to zip tie anything. Mind that I did not remove the existing fans, just used thermal interface material on top of the existing heat sink, add some salvaged aluminium passive cooling sinks on top, and then on top of that, have the fan screwed into an Amazon shipping box so that the fan blades didn’t hit the sinks.
