Updated - MumboJumbo's Slim 7 Segment Display [TUTORIAL] 

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So I finally managed to make a tutorial that doesn't suck, even though it's quite long.
You can substitute empty cauldrons with any block that doesn't interact with comparators, like wool, wood or glass.
It's made to be stackable, and this is the simplest design that can do so - if you want to put 2 or more next to each other, don't leave any spaces between them, just place them and then cut any wires that join together.
If you don't like the original position of the button, you can place it somewhere else near the display, and then wire it down to the original input block.
NOTE: This is not the best one, PeterjiangTW has done a better one.