Interesting design. Too bad it does not include a sight glass to see the oil level. You'll have to fabricate your own mounting bracket, as it does not bolt into existing battery box or oil tank mounts.
Cognitomoto also makes a replacement oil tank designed to mount on the triangle subframe rails for $310.
Round vs. Rectangle. It all comes down to what pleases you more aesthetically and your ability to adapt the Carpy tank to the frame.