I would have thought the rack would hold more, I have one that is rated 100 pounds. So here is another option, search Google images for "bobber seat for bicycle". You should get pics that will lead you to this link.
http://ratrodbikes.com/forum/index.php?threads/bobber-seat-conversion.58486/ The pics in the link are screwed up by Photo Bucket, so you have to use the Google search pics. I think it is doable with a little welding, or maybe even clamps. The clamps that hold a chain tensioner might work, or maybe a modified to clamp across both tubes.