I figured out a way of making sign art on servers that support color codes, such as Hypixel. While trying to make these, I've discovered an issue with the Unicode block elements. Block elements are supposed to be monospace, but they don't render as monospace on signs. All four quarter blocks and the left half block render without a gap. For example, the first image is what block characters look like in-game, while the second is what they're supposed to look like. Here are the characters used, which are spaced correctly here as well:

&1▙▗▌▚▘

&3▌▘▌▐1

&1 ▜▘▌▌▞▘5

&3▐▁▚▘▚▖