This is version 2.0, built it for my 580B - maybe you could scale it down for your smaller tractor. This one's 6'x8'x2' high.
Frame is 2x tube, .120" wall with 1/2"x2" FB at stress points. sides are 16 ga., bottom is 14 ga. over tube lattice.
The brush bucket actually slides into the loader bucket til the back touches the upper side of the loader bucket, this puts the bucket bottoms on the same plane.
The front lip I added 2 pieces of 1/4 x 4" FB, bottom FB welded to bottom corner of the 2" tube and upper FB welded to top corner, forming a triangle "mini-ramp" - initially just to make a little ramp so our big rubbermaid cart could load into it easier, but found I can move an entire compost pile using it as an extension of the loader bucket.
It also doubles as a work platform, I can put a small genny in it and do tree trimming with an electric pole saw. A lot of the time I can trim, get wifey to play "elevator girl", then just drive to the burn pile and dump. Learned a long time ago that material handling can help or KILL you, my method allows handling ONCE while trimming.
You might need to figure out a different mounting, I decided about 10 years ago that I was NEVER gonna be satisfied with just hooks on the bucket; hence the 7 receiver tubes (and an ever-evolving collection of "tinker toys" to plug into 'em)
Scaled down for your smaller tractor, I'd probably look at 1-1/2" x .095" wall tube frame, maybe 1/4" FB at stress points, 16 ga. skins and at least one triangle per side of the frame. I couldn't find any lift specs on your Yanmar, but I'd probably go about HALF the length of my version. If you made yours fit inside as I did, all you'd need for mounting would be a link from the add-on to the two outside top corners of the bucket in your pics... Steve
Oh, my mount takes less than a minute on or off; curl bucket down, inch forward til pins contact those big hooks, curl bucket up, brush bucket slides into loader bucket, done. Reverse process to disconnect. Lets me "park" the brush bucket anywhere for wifey to work a particular area, then yell at me to pick up the (full) brush bucket and drive to the burn pile - all the time just sitting on my butt and wiggling levers :=)