Headcanon that after Matt is rescued, he starts travelling the galaxy, fighting on behalf of the rebellion and doing whatever he can to screw over the Galra. But he also develops a habit of picking up souvenirs on each of the planets and moons he lands on - just little stones or leaves or sticks, whatever he can find - whenever he thinks of Shiro or has a moment where he wishes Shiro is with him. He collects and keeps all of these little trinkets in a box that he carries with him, promising himself that one day he’ll get to show Shiro his collection.
Then he and Shiro are finally reunited. Matt brings out the box and explains what it is before handing it over. Shiro thinks it’s adorable that Matt made a little collection for him, but then he opens it up and is absolutely gobsmacked when he looks inside.
Because the box is filled to the brim and there have to be hundreds or thousands of little rocks and shells and plants and twigs and just so many things from so many planets and this is how often Matt thought of him and this is how much Matt missed him this is physical proof of Matt’s feelings for him and he’s just so overwhelmed he has to put the box down and rest his head in his hands and just breathe because holy god, Matt.
And then Matt has to sheepishly admit that this is just the first box.