When I was considering some ideas on my build, one idea I had for an oil tank than might work on your build. I was thinking of puting it in the rear fender. Not sure how thick because I didn't do the math but it basically involved welding two fender halves, with a fill and vent tube on the inside. I would have put the hose connestions on the bottom and a small flush fill cap. Then watch people wonder where the oil tank is.