Okay got some spare time this week, here it is. This is only the "locking system", not the whole attachment.
The mentioned measurements are the ones I used, You can change them of course to your own needs.
For example, I had some scrap left, so I used a rod/bar of 16mm for the handle, If you have a piece of rod of 20mm laying around + some pipe that fits snugly, you can use that of course...
If it still is not clear, don't be afraid to ask more info.
View attachment Manure shovel - Locking system.pdf
edit: these 2 images above are gif's, click on them to see the animation.
