Heatsink/Thermal pad between NVMe drive and motherboard.

Good evening, everyone.

Another long one, I'm afraid...

In the near future, I plan to swap out my stock 512Gb NVMe drive for a Crucial 2Tb drive, on which I will have cloned my boot drive.

I removed the current drive while in the case the other day, to see exactly how it was situated. Oddly, there was a small, square spongy pad under the contact end of the drive. Reinstalling the drive involved inserting the contacts, and pressing down the drive against this pad to fasten the screw at the opposite end.

I now know that this pad is a standard thermal pad (20x20x5.25mm) for this system. 

But I am a bit confused and concerned. Why so thick (though it seems to set the drive level on both ends)? And if a thermal pad, why just on that first 20mm? That would seem to place a bowing effect on the drive as the pad was compressed into place.

All this is leading to my upgrade concern: when I put the new drive in place, I would like to place a "normal" (120x20mm) thermal pad under the drive (the top has a heatsink pre-installed). Will I need to build or layer something 5mm thick to allow the drive to sit properly?

And is it even a good idea to transfer heat to the motherboard? 

Better to leave the 20x20mm pad in place? Is the smaller size designed to allow airflow between the other section of the drive and the motherboard?

Ideally, I would like to layer two 1.5mm pads on either side of a 1mm thick copper plate, for 4mm total.

I will do a dry fit of this setup and see if the drive can be installed normally and without stress. 

I guess I just need to know if this idea would do more harm than good. 

The Dell designs are odd, but the systems work. 

Any wisdom regarding this issue will be greatly appreciated.