Here are 2 iphone cases with a great Knuckles theme. They're red all over, and nothing but Knuckles. The first has various 16 bit sprites of him doing different frames of animation from Sonic & Knuckles game. The other is a modern stock art of him with a quite-close crop, letting the character take up the entire case. The art is the 'fist out' Knuckles that's not particularly over used in 2018. While the first case is a bit busy, the second one has a nice impactful design.
*Why are these things only ever for iphones? Because the models don't vary much so they're easy to make and keep on the market while not spending much on new molds to cast the cases.
*Why are they so expensive at 35 dollars PER piece of plastic that makes up these cases? There's likely no answer for that one....oy.
These are both Sega Shop Online items that were added in 2018.