Major "End-Game"/"Post-Game" spoilers, obviously.

After completing all 7 endings, you can throw the Kid Wizard book into the cat clock behind the TV set. This transports you to a "cat clock dream world".

In said dream world, there's a ring of 12 cat clocks (representing each hour). You can grab the giant clock hand at your feet and use it as a very long handle to carefully nudge the clock hands on the cat clocks to point the 12 clocks to their corresponding times. When the hour hand is pointing at the correct value for any given clock, its eyes light up.

When you are done completing that sequence, the cat with the broken clock hands has its eyes change to "human" eyes, and it gives two hints: "light the way to 11, kid wizard" (paraphrasing), and: "use the minute hands 123ABC".

You can record the minute hands as pointing to the following sequence:

4, 2, 1, 4, 4, 1, 4, 2, 1, 4, 11, 9

These presumably correspond to the letters:

D B A D D A D B A D K I

Which "rearranged from 12 to 11" spell out:

BAD DAD BAD KID

Then, touch the Kid Wizard book to the flashlight (the big lantern-style yellow flashlight). You can find this flashlight in one of the later hours of the day, or, toss the Kid Wizard book into the computer and use the flashlight there in the secret room which has all the game objects.

This will transport you into the playhouse out in the yard, inside of which is a ouija board. The letter range on the board goes from A through K (corresponding with the 12 hours).

Tap the glass on the ouija board onto the letters around it to enter the above code ("BAD DAD BAD KID").

After entering it, there's a bolt of lightning, and it takes you to a room claiming you've found the real ending (but "don't believe his lies"... the fridge sticker tells you that the monkey lies, and it's true).

Behind the podium the monkey is sitting on is a hidden copy of the Necronomicon. Throw it into the door and there's a bonus room with what looks to be a possible teaser poster for Stress Level Zero's next game, and a cool concept art piece for this game and for what is presumably that next game.

There's some binary code on a machine in the concept art poster, I didn't try translating it. I also think the concept art might be representing the kid as a grown up, or the next generation of player? Fun head trip.

Then you throw the wizard book into a bloodied "exit" door, and SteamVR exits the game.

Something tells me that's technically the final ending to Duck Season.