1BADk9,
To make an oil cooler out of the stash tube could be done, however it would be probably cheaper and easier to just buy a new one.
The way would be to built a plate or tube bundle to fit inside the stash tube and have the inlet and outlet pass through for hooking up the hoses. IMHO, a plate type would the easist to do, that said, the plates more than likely would have to be machined press or milled, solder in a tight non-leaking stack and then inserted into the tube with a fairly close fit and then such soldered against the interior of the tube to insure proper transfer of the heat away from the stack. Paste solder and heat would do that easily for the conductivity.
The other option would to machine such to very close tolerance with an interface fit and then freeze the stack and insert where there is good conductivity.
Allot of hassel, but if one has the time, skills and tools-could be an interesting build and project.
That all said, IMHO two-coolers is not needed but would not hurt. I have road over 500 mile trips that were way into the 90's without one and never had an issue as long as I was moving. If you are not moving air across, the coolers will do very little (though still some). Good syn oil and fast riding will keep you happy as much as two coolers would.
Good luck and let us know what you end up doing.