You misunderstood my answer. Yes the 690 Enduro and SMC use the same tank.
The tank of the 450 Rally has the same basic shape as the 690 E /SMC, the only difference is that it is a bit more filled out on the left side in the area where the 690 has the muffler. The rear fender, the filler, the fixings to the frame, the cutouts on the bottom (fuel pump on 690, fuel tap on 450) are all the same.
If 12 liters is enough for you, you should be able to find a used 690 tank a lot cheaper than you can buy a 450 Rally tank. Then you can also tuck in the muffler so it is not as prone to damage as when mounted lower. I think a friend of mine has one, if you want I can ask.