Important notes:

• This trick can only be used if you have unclaimed rewards in The Grand Tour.
  If you've already exhausted all your reward claims, you can't use this method anymore.
  
• Only certain challenges, usually the most difficult ones, will grant you extraordinary items.
  
• You might be exposed to a little bit of French in some screenshots, I sincerely apologize for that. ♥

You'll want something like this:



And not like this:



(Ridge Over Troubled Water is just an exemple of challenge granting extraordinary loot.)

The Descenders save file is a bit unusual, as it's partly stored as a registry key.
Which is the only reason this trick works so well.
The process is quite simple:

• First, you need to find the registry key that concerns us.
  To do this, press the Windows key, then type regedit and launch it.
  
  
  
  
• The key we're interested in is located here:
  HKEY_CURRENT_USER\Software\RageSquid\Descenders\
  
  
  
  
• Right-click on the Descenders folder and export.
  
  
  
  
• Save the file somewhere easily accessible. (I call this file "Descenders", but I don't think it matters anyway.)
  
  

With all that done, the trick is pretty simple to execute.

• Launch Descenders and claim all the rewards available in The Grand Tour. If you don't get the item you want, don't worry.
  
  
• Close the game.
  
  
• Double-click on the "Descenders" file you just created to edit the Registry.
  
  
• Restart the game.
  
  
• You'll see that you can once again claim The Grand Tour rewards you just claimed, AND that you still have the items you unlocked before closing the game.
  
  
• You can now close the game and click on your "Descenders" file as much as you like, until you get the item you want, like the elusive whoopee cushion, for example.

This is the first guide I've ever written.
After the frustration of grinding for hours for the whoopee cushion, I finally discovered this method.
I haven't found this trick anywhere on the web, so I had to share it.

I hope you find this guide helpful, and please feel free to comment if you think any section could be improved.

Take care, and I hope this will help you 100% this incredible game with a little bit less of frustration.