Those who have lighter benches may have this same problem; when planing the edges of long boards, the bench lifts on the tail end.
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This used a $3 bag of cement (80 pounds) set into an old soap bucket to keep the end from kicking up.
One edge was cut out to rest on the bottom cross member. The hole was filled in with a piece of scrap and the cut out piece with tape before the cement was mixed inside the bucket.
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It works great. 60 pounds of cement would likely do the job, but the 80 pound bag is only about 50˘ more.