Nice game, I managed to beat all the levels, the last one took some careful planning to complete!
Portals feels a bit weird because they look like they are on the ground but are actually suspended in the air and reaching the portal by ground is not enough to win. Also the boxes moving and squeezing each over in planning phase feels a bit janky. But these are some nitpicks and I enjoyed playing the game, the puzzles are good and require creative solutions.
Another thing I would change is the control scheme. Instead of using drag and drop, I would bind 1,2,3,4 keys to empty box, explosive, timed and black hole respectively. You hit the key then click around to place chosen boxes, right-click to remove boxes. I think drag and drop is good for one time actions but not for repeated actions.