Cut cardboard/carboardbox to size?
A few holes for the cabkes as needed. Job done. Possible bly some air holes/slits at either side.
Add paint if you want to change the finish.
Or go fancy and find a MakerSpace and get making.
Though I will add, I would lower the POE hat to be closer and 'inside' the Pi, then mount the top hat further way from the PoE hat, less magnets and electromagnets, more room for the fan to work.
If I remember correctly it is designed to sit inside the Pi area.
You may be able to make it more compact, or the same with more air room