Just curios is this Tom?There is apost on here on how to convert subframe into spare fuel tank I believe Allen from Australia did this and also added a breather to back side of sub frame to allow fuel to flow in better and more capacity.I use a 6 liter fox fuel bag if going far and empty it as soon as possible also giant loop makes fuel bags.
Tried it on my 12 570 many years ago. Found the area around the grab handle holes was very thin and Leaked. Gave up and brought a 70 degree subframe tank. Sorry not much help here.
The subframe holds exactly 4 litres. Yeah some subframes can be a little suss while others are fine. Worth the try though as its cheap and quick. I put a screw right up in the back corner as an air bleed. Works brilliant. I run the air vent all the way up to meet up with the main tank also a vent for my bash plate tank that also runs 4 litres.