Quick attach clamp on bucket forks should have a "c-clamp pad" design. Look for the largest c-clamp pad you can find. The pad is on the bottom or the base of a handle that screws down onto the top of your bucket. A large pad will make a more secure fit and will not cause damage to your bucket. Some brands of clamp-on tractor forks either do not come with a pad or have a pad that is either too small or is non-swiveling which will not create a secure fit and in some cases can actually damage the bucket.
Clamp with T handle and large pad - a good design
Damage caused by missing swivel pad
A well made clamp-on tractor fork attachment will come with a sliding T handle or other type of handle that allows you to securely tighten the clamp to the bucket. The clamp should also allow you to get the tractor forks close to the sides of the bucket. Some clamp-on tractor fork designs use a welded-on handle which will not permit you to securely tighten the clamp or move it out to the sides of the bucket.