Doing a quick google search yielded a number of thumbs out there and you would need to find one the right size. You could maybe even have one custom made for not too much I would think. They are not very complicated. You do need to change the pivot pin of the bucket to a longer one to accommodate the thumb hinge point. And I would stay away from the clamp on type of thumbs since they can sometimes create damage.
I would not go too much smaller than a 12 inch bucket. Depending on soil type you may find that a smaller bucket just gets plugged up and then you have to use a spade to clear out the bucket every time. Kind of defeats the purpose.
I hope this helps!!
Here is a link you can check out.
