I don't know that you'd necessarily need plans for a box. I think the only thing you'd need is the dimensions of the urn that would fit inside. You could make the box any type and style that suited your (and your mothers) taste.
Much like differing tastes in coffins, it could be a simple pine box or a very fancy one with exotic woods, inlays and brass hardware. Talk with your mother (and other family members) and see what type of box they think appropriate.
"Good Behavior is the last refuge of mediocrity" -- Henry S. Haskins