I have to eat crow.
The picture with the cat does not show the step in, but your cad model does.
The arm included with my DA85 will not let the arm come out using the side to hole placement you have.
Must be changes all the time.
The cad model does not have a opening to adjust the needles, do you just cut the stack where you want?
Not a problem to do so.

None the less I am glad someone came out with a solution.
I am one of the old farts that makes a lot of stuff and spends the saved money on other items.
I hope you do well on sales!

Tony