That looks great! To be honest the paper diffuser still works pretty well :)
That's awesome. The diffuser isn't bad - you need some kind of diffuser, otherwise you can see the individual colors in each LED (like red/green dots inside the yellow). Vellum from art store is considerably better than straight up paper. It's a little clearer, looks more even, and less obviously cheap.
The diffuser (and frame around it) also lets you hide some super bright lights along the perimeter, which you could do a light show with (ie, a few bright flashes right after a tetris, making it harder to see the game action - this is legit tactic in arcade games, used very effectively in pinball)
Really, no need to excuse your paper difuser - your tetris gameplay, on the other hand... ;-)
For this size, a 3d printer comes in mind. Or use a prismatic acrylic sheet with corresponding spacing (1/n of the led spacing). Should be nice without a grid.
I build Tetris on Espruino and output it on a 16x8 RGB matrix. I'm controlling it work some push buttons which works well but I might build something better.
You can download the code here, it will work with most displays just change the Graphic settings at the top. There are still a few small bugs and lots of features I want to add.
Here is a quick video showing the game play (excuse the crappy paper diffuser but my camera can't focus on the LEDs without it)