This is a small bracket to hold an ND filter for the Zerotech Dobby. Care must be taken when positioning the bracket to ensure that it sits squarely over the lens and wholly within the lens assembly. The best way to do this is to use an impact adhesive to give yourself time to move it into position. Stick a piece of ND filter gel onto the back of the inner frame and trim to size. That's it!
Note: I printed this using PLA at 220 deg without any problem but the side walls are thin (about 0.5mm) and so you need to make sure the layers bond properly. I did also try printing using Colorfabb nGen and the side wall peeled away. I then kept the nozzle temp high (220 deg and the fan at 20% and that seems to have worked.
EDIT - I've added a new stl file (and Openscad file) called dobby filter2. This is a slightly wider bracket so as to be able to accomodate a slide into which the official Zerotech ND filter can be stuck.
In my view this is a much better arrangement because the Zerotech filter is far better quality than a bit of ND gel which tends to lose its coating after a while.
So, if you have the Zerotech ND filter, use dobby filter2.stl.
0.4mm nozzle, 0.1mm layers
PLA at 230 deg with 50% fan from layer 2 onwards worked well for me.