I think I could build something like what you are asking for out of 3 or possibly 4 8-foot 4x4 posts.
First, tuck a 4x4 along each side wall of the bucket (should stick out maybe 6ft past the bucket) and raise the bucket until the boards naturally tilt and the ends of the boards are floating in the rear of the bucket. Once you get the angle about how you want it to achieve that downward angle you mentioned (to compensate for the ‘rollback’ of the bucket as loder lifts), then push the boards in until they’re hitting the back of the bucket. Then measure the distance vertically down from under the top lip of the bucket, down to the 4x4. Cut a 4x4 to that ‘height’ and attach it the 8ft stick so that the short vertical 4x4 piece is entirely ‘tucked’ under the top lip of the bucket (flushed to top edge but not protruding out past top edge), but definitely touching the side walls. This should make it ‘self centering’ in the bucket width wise. Make a mark on top of the bucket where the center of those 4x4s is under that top lip. You can now measure the width between your two 8ft sticks and cut a brace to fit between them and ‘set’ that width at the bucket end. Out at the front end you will attach a ‘push bar/bumper’ of 4x4 probably 3-4ft wide (make sure its a good bit narrower than the bale youre pushing so it wont snag any neighbors) and screw it to the front ends of those sticks, and at that point you should have a rigid assembly a bit like a capital H but with a bar across the top like a T.
Ok, so drill/torch/plasma a 1-1.5” hole where you marked the center of those vertical 4x4s on the top lip of the bucket. The idea is drive a big ol lag screw down into the tops of those 4x4s leaving them at least 1-1.5” ‘proud’ so that when you drive up under the end of this thing and lift the bucket while inching forwards, those big bolt heads will find and pop through those holes, retaining the thing from sliding out the front of the bucket until you drop it back down close to ground level. In my mind you should be able to pick it up and drop it from the tractor seat.. in theory! The only thing left to add is to brace between that short vertical 4x4 and the back end of the 4x4 which hits the back of the bucket and gives all the pushing force. The vertical 4x4 is just setting the ‘droop’ angle and keeping the thing from sliding out by hooking some big bolt heads on the edge of some big holes, but it should never be providing the pushing force which should come from the end of the 8ft sticks contacting the rear of the bucket. If the vertical 4x4s are short enough you might not need that bracing, or you might be able to lay a 4x4 piece down horizontally (4” tall..) on top of the 8ft sticks and leave the excess length hanging out the front where it wont touch anything, and put another lag bolt through it there so youve got 2 bolts in shear vs a tall ‘column’ in bending that would need shoring up with a brace.
Like i said, i think i could build it out of 3x 8ft sticks and a bunch of large lag screws. At that size of lag screw i personally would pre-drill those holes so the screws are less likely to split the 4x4s.
Here’s a shitty pic i did on a iphone 6 screenshot that shows the ROUGH angles of the bucket end of things.
Good luck!
