23/02/2020
OK, I have completely re-designed this Cat Toy from scratch. The old one, the walls were too thin and it was a bit of a pain to put together. In the new version I have thickened the walls and made it a lot less complicated.
Instead of it being two "L" shapes, it is now a box and lid so assembly is a piece of cake. Also, I have intentionally left a bit of empty/unused space in the bottom of the box so that you can add some sand or rocks or whatever you like to add some weight to the box so the cat doesn't knock it over or bat it across the room.

PRINT THE PINS WITH 100% INFILL. For my test print, I did all the parts with only 10% infill so the dowel that pokes out of the hole snaps off VERY easily. Mine broke simply by assembling it. Re-printed with 100% infill solved that so now your cat shouldn't break this just by looking at it haha

IMPORTANT NOTE-
I designed the box so that it can be scaled up or down to suit your needs. If you shrink the box though, the filament holes will no longer fit 1.75mm filament so you may have to drill it out or print a pin which is easy to do.
Same goes if you enlarge it, a filament pin will work BUT they will have to be glued to the pin so they do not wobble themselves out.

19/07/2019:
Just a quick design I threw together for a cat toy.
The pegs on the push pins are large enough to glue feathers or toys etc to.