This bird feeder is designed to mount with suction cups on the outside of a window that swings vertically when opened, making the refill very simple from inside. Here is a short movie showing the function:
https://www.youtube.com/watch?v=DhzEpi22kVw

It is designed in onshape. Feel free to open and improve the CAD here:
https://cad.onshape.com/documents/98e8167d4bba4c7daf8bb67a/w/802036990876fcccc2b15aab/e/740f587c05faee60a913bcad?renderMode=0&uiState=638cd3cacc89dd6a71fdba98

After printing, thread the holes for suction cups with an M4 mm tap, to make insertion of suction cups easier.

3 pcs 50 mm suction cups with M4x11 mm screw can be purchased here, article 5010:
https://www.pencraft.se/butik/produkter/sugproppar/3010-4010-5010/