I did build a sunshade for my i-pad mini.
Used materials: 1/2mm photocardboard, black gaffer tape, sheet of pre-glued velvet.
I made it out of 3 pieces. I didn't use tabs or glue, but I used gaffer tape instead.
The top and the back is a folded panel. the side panels are separate.
The inside I lined up with black velvet.
A rectangular opening at the top to let the holder trough.
To have it held - cut an under tablet panel same size as tablet .... then cut out a slot at top and bottom edges to let holder insert ... cut similar from back vertical panel.
The slot in under tablet panel must allow holder to grip tablet.