A while back, I bought a Gainward Geforce 3 GS on eBay, but it didn't have a cooler. I have an old Thermaltake Crystal Orb and an all copper passive cooler that I could use, but I decided that since this is a hobby, it makes sense to try something custom. I had previously seen a few prototypes on this forum using Pentium M coolers. I like the look of them, so bought one and spent time planning out how to mount it.

Here are the parts that I eventually settled on for this small project:
Evercool Pentium M cooler
4x M2 25mm Cap Head Screws (22mm would be ideal for reasons you'll see below)
8x M2 Washers
4x Nylon Locking Nuts
4x 4mm diameter plastic spacers
4x M2.5 plastic shoulder washers (optional and maybe not recommended)
4x Springs--5mm outer diameter, 5mm length, 0.5mm wire diameter
1x 3pin to 2pin fan converter
Teflon Tape (optional--I was slightly worried about the screws connecting electrically to the mount holes, which have metal eyelets)

Mounting holes
The way that I planned to mount this is not via the two primary mounting holes to which a typical cooler attaches. I noticed on Geforce 2 and 3 cards, there are four smaller holes in a square pattern around the GPU core. You can't fit an M2.5 screw through these holes. The biggest screw you can fit through is an M2 screw.

Modifying the cooler
I planned on mounting the cooler through these, but they are slightly offset from the mounting holes on the cooler. I needed to bore into the side of each of these holes in the direction of the center mass of the cooler. I used a ruler to draw some guide lines and planning on boring about 2mm in for each hole. Now, importantly, I tried to do this initially by screwing the cooler to a piece of wood and using a Dremel. That was a huge mistake. Every time I moved the boring bit into the copper, it bit, jumped, and scuffed up spots on the cooler that were unintended. In fact, it seemed to only bore into spots I didn't want it to. You can see some of the damage in one of the pictures. Ultimately, I decided to using a set of needle files I have. My needle files are #4 fineness, which is not ideal. I wish I had some #2s, because it took almost 2 hours of filing with my #4s, which are more designed for jewelry and other fine work.

After I finished filing, I lapped the surface of the cooler. This caused some discoloration, which I tried to clean off with some mild acid (which did nothing) and I eventually moved into "don't care mode" and simply cleaned it with some alcohol.

Installation
I applied thermal paste I had on hand from a recent system build and mounted the cooler as shown in the pictures. The only thing I'll say about this is that it's easier said than done to hold onto a card, steady an allen wrench, and tighten a nut driver. I need a third arm for this stuff (or you can hold the allen wrench in your mouth like I did).

Problems
First off, I'm kind of ashamed to say that I went through 5 separate rounds of ordering because (a) I misestimated sizes several times and (b) getting the spring tension just right is a trick. A smarter person would have gotten this right on the first try (and you can imagine that ordering these parts each time in small quantities is expensive). This is partially why I made this post. It might help someone.
Second of all, you can probably see what might be a future problem. 25mm screws will very likely hit the cooler on a retail Slot 1 Pentium III CPU. I have a Socket 370 CPU on my testbench right now, but I held an SL4KL I had within reach up near the slot and it looks like it will hit it. Shaving 3mm off the screws would be enough.

Other Notes
The reason I spent so much time testing different springs is that I was worried about applying too much pressure to this antique and breaking the BGA joints underneath the chip. Ordinary coolers from 2001 would have been held at two mount points with weak tensioners. This cooler is pretty heavy, so it needs more tension, but it also has four mount points, so you have to be careful how much pressure is on it, IMO. I think the springs I picked are perfect, but I was pretty close to settling on some 0.6mm wire diameter springs too.

I'll update this with more pictures in a couple of minutes.

Edit: Added 3pin to 2pin fan connector to the parts list

Card as it arrived from eBay seller

Cooler with guidelines drawn

Cooler Top

Cooler after lapping

Parts