You think the bases need to be thicker material? I'm about to make some...
On my next round of bumps I plan on making the rubber or poly mount to the spring plate and have a flat plate at the appropriate distance come down from the frame. Not sure if that make sense but the part coming down from the frame would be welded on instead of bolted. I think the thinner base (3/16" I think) I'm using is a culprit along with the toyota factory bolts I used for the frame holes. I've already sheared one of those off so perhaps some metric 10 or 12 grade would hold up better.