With the plane added delete 1 edge and bevel the side corner and with the bevel circle i have bevel it. Problem it bevel but the center part is flat and the both end are rounded like a tube. I have set the radius 1 yet to cant find the result to be fully tube.
The bevel object, used to bevel a curve, follows the curve's scale.
So if the scale of the curve is not homogeneous, the bevel object will appear unevenly scaled itself. To correct that, use CtrlA then 'scale' to keep the shape of the curve, and set all scaled directions equal to 1.