Greetings All!
I made the mod, and here are some pictures and description of what I did. I hope they will be of help to someone.
I made the initial holes, but the sub-tank 90 degree elbow was not where I expected to be, and I did not liked the rear shock spring clearance.
So, I decided to plug the hole and make a new one, but this time on top. I drilled with much smaller drill and used a hot air gun to enlarge it, so that it's more like vortex shape. That way the thread will be stronger, and there is a nice place for o-ring.
I added fuel resistant hose to the 90 degree elbow so that it goes to the lowest part of the sub-tank.
Here is how it looks on the bike:
On the fuel tank I put fuel ****, so that I can enable or disable the sub-frame tank. Note that the orientation on the picture is not final.
Here final orientation:
Finally everything is in place. You can see how the fuel **** is accessible from behind the shock absorber.
I routed the breather to the fuel tank breather:
After I filled with fuel there was a leak somewhere between the tanks. I removed the sub-tank with the fuel in it and find out that there is a week place in it!
The material was very thin, like a garbage bag. On the other side it was also thin, but acceptable.
As there is no place to make big hole and plug it, I had to drill the sub-tank all the way to the other side, where the under-seat grip is located and make "2-way plug".
The o-rings are not yet mounted to the plugs.
Here is how it looks back side:
And front side:
I tested the setup last weekend and it was holding nice. I noticed that there are some fuel in the back, that is not used, unless you tap the front tank breather, so that all the air comes from back to front, that way all the fuel in the back tank will be used, before it starts to use fuel from the front tank.