Nice effort - Your home made bell target works well.
Those small rear diopter sights that you have fitted on your HW rifles work well on the HW springers - I used one on my HW30S for a while, but changed to a s/h Anschutz when my neck started to give me some gip and a friend gave me his old sight - same result - but being a little higher I found it slightly more comfortable.
Bob.