Step 1: Make sure you have some keys available to redeem in your Inbox, dont redeem yet. The more you have stacked, the better.
Step 2: Disconnect your console from the internet, wait for the Connection Lost message and redeem the keys.
Step 3: Save and quit to the main menu and load back into your save.
Step 4: Go back to your Inbox and you will have the keys you just redeemed and you will still have the message to redeem them again. To repeat this over and over, just simply keep on save & quitting, loading back in, then redeeming the keys again.
Step 5: Once you have the amount of keys you want, save & quit again and then reconnect your internet.
Step 6: Spam the chest.
This is best for new players to the game or new character runthroughs, you can also just sell the items if you dont want them which will make getting those SDUs a little easier. Hope this helped anyone out and please leave any questions in the comments. Cheers