Bearing blocks are probably the trickiest part to design and machine of a WCD. At this point, they've become pretty standard for various reasons. Many teams use something like
this.
As Adam said, the relatively poor tolerances in tubing makes bearing blocks on the inside difficult. The tubing from McMaster-Carr has tolerances of .044" for the outside dimensions, and it's probably much worse for the inside.
One benefit of the standard method (linked above) is that you can put a CAM on the pointed ends for tensioning.