This may or may not be of help for anyone else installing the TRM tanks. There are always multiple ways of doing something, and this is how I did it:
First I lined up the tanks on each side of the frame how I wanted them and taped them tight across the frame so that they would stay put. Then, using a triangle, I marked as best I could, where the perpendicular tangents of the tubes to the tank where the lower and upper mounting points intersected. After that, I took the tank off and assembled it, also tightly wrapping tape around the tank near where I would need to cut so that I the halves wouldn't try to separate when the drill went between the halves. So, I got the hole saw and a 10 or 12" long 1/4" bit for it. I found the center of my head tube marks (which actually measured out to almost the exact length as the diameter, which kind of surprised me) and drilled the top with as best aim as I could in the direction of where the center of the bottom hole would be about 1 1/2" forward or so, but I didn't drill into the bottom. I then did the same with the bottom as I did with the top. Finally, I used the two 1/4" holes as guides for the hole saw thanks to the long drill bit. I cut the top most of the way, but not completely (so I could use the top 1/4" center hole as the guide for the hole saw on the bottom), then flipped the tank over, drilled the hole in the bottom, then finished the top hole.
The other parts were pretty straight forward work with drill bits and files. I did end up having to fine tune the head tube holes a little as this isn't an exact science and the tanks sitting alongside the frame will be oriented slightly different when they are actually installed as the frame tubing variances change the angle slightly (FWIW, I had to rotate my tank up slightly at the back, meaning I had to open the top hole a little at the rear and the bottom hole at the front, but it's hardly noticeable and will be fixed later).