Have you checked out the BxPanded Back Hoe Thumb?
I have a Kubota
BX25, which is a small tractor similar to yours and have purchased/used the BxPanded thumb.
When I first purchased my tractor, I bought a Wood's thumb off Ebay, but it was not really designed for a small tractor.
Plus the teeth of the thumb did not sit perfectly/nicely between the teeth of the bucket, where the BxPanded thumb did.
Prior to buying the BxPanded's thumb, I also looked at Bro-Tek's Thumb, it uses the 'Top Link' from your three point hitch.
Which I questioned if it would hold up/withstand the stress/punishment.
So, I purchased the BxPanded thumb two years ago and am very pleased with it.
It has never let me down.
Below are links to all three back hoe thumbs:
1) BxPanded Thumb
Barracuda BH Thumb
2) Woods Thumb
Kubota BX 25 BX 23 BT610 BH75 and Woods Backhoe Thumb | eBay
3) Bro-Tek Thumb
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Hope this helps.