The attachment method looks very good and solid with the chain tensioned with the threaded rod. the big issue is what to make the forks out of, something that would hold up to rigors of fork use.
those materials are fine for a scut like my gc2410. btw, there is a chain bracket on each side of the bucket.
the flatbar is the same thickness as the cutting edge on the bucket, so there is one scale guide. far as the hss, a person could bump that up in 1/16 wall thickness increments, or 1/2" increments in cross section size relative to the lifting power of the loader or the application.
weight is an issue with the smaller machines and i wouldn't go any thicker/larger for those. even with the listed materials these are fairly heavy and reduce the overall lifting capacity accordingly. hss is pretty structurally strong stuff, much stronger than pipe or angle iron.
it might not be a bad idea to figure the running weight of the materials and then design the unit to weigh no more than a certain percentage of lifting capacity of whatever loader it is going on.
that small square image at the bottom of the threaded rod is a piece of keystock to keep the rod square to the mounting plate. i don't have one of those on the prototype and the rod gets pulled crooked by the chain.
there may be a better way to connect the bottom of the chains to the tubing, but the way shown is quick and easy.
the bevel being on the bottom side of the fork sometimes makes it a little bit tougher to pick things up, but it prevents a fork or two from heading south on their own accord. again, you can build the bevel either way depending on your application.