Since display entities were added i've observed the way text displays allow you to make a perfect square with a color of choice, but what does this mean? Minecraft is a simple game in terms of graphics, with text displays it's possible to re-create 2D textures, in the case of an inventory I suggest hard-coding it once and just using that as the gui, then you can make the player's camera be stuck at the point you need it at (either through tricky mounting or through packets) and spawn entities in the way you need.To go along with this, you could have a text display with a transparent dark gray color behind the fake gui to darken the rest of the worst.I also suggest looking into doing this with packets as doing so otherwise might cause some strain on the server when a lot of people are playing.