We too have B30 and have replaced the original CQR which proved unreliable. I went for a Rocna and although not tested in severe conditions have been pleased with it. You can download full sized templates from the rocna website to make a model and check it fit the bow roller with clearance for the hull.
I bought the 20kg version, on the basis of bigger is better and it can come with us to our next, bigger boat when the time comes.