You can redeem 4 Silver Random Boxes every 30 minutes (not necessarily in game) by pressing this button on top of your screen.


As you can see, you can keep track of the counter right below it.

Here's what you have to do every once in a while, at least 30 minutes apart:

1. Open the game
   
2. Redeem the boxes
   
3. Close the game right away

Sadly yea, in order to make it work you have to leave the game right away because random boxes don't take long to show up.

The less boxes you accidentally make show up before closing the game, the earlier you'll be back at playing the game normally.

Stack as many Silver Random Boxes as you need to unlock these achievements, respectively:

• 25 Silver Random Boxes
  
• 50 Silver Random Boxes
  
• 100 Silver Random Boxes

To reach 100 unspent Silver Random Boxes took me less than a week.

The biggest struggle is to avoid playing while saving for boxes, but you can do it!