FYI, you don't really need the deep pour epoxy to coat a sign. That's more useful for river tables or filling large gaps/holes or molds and such.
I guess you could make a mold of a Q or something, and make multiples with the epoxy, but that's a lot of work and would be rather costly. I had a few ideas of my own for signage back in the "glorious" year of 2018, but I was broke AF, and somehow even broker now. Which, in case you're wondering, everything in life costs money, so basic survival is still kinda a main priority for me.
But I do like where you're going. I just felt the need to mention the little bit I do know about epoxy, because it's not always that easy to work with, can be expensive, but does look amazing if done correctly. Most tabletop 1:1 mixes would likely work for a decent coat over an OSB sign. Although, the glues from the board can cause problems if it's just bare wood. So, paint it well to be sure it's sealed. And if not, start with a light seal coat first, then let it dry, then do a heavier coat. Although that would take extra time. It'd be a fun project, I'm sure.
I'll add the one idea I had converted for real world signage. I stumbled across this magic trick one back in October 2017, not long before first Q post ever. This is a super cool way to make heavy duty stickers or decals, which can last quite a long time and stick to most construction surfaces. Although, I will say that the sticky part is kinda like tar, so be careful what type of surface you put it on. Could go over OSB or plywood, if you wanted to pre-make signs.
First, take Protecto wrap, which is used around windows and doors and can be found at [your local hardware store]. It comes in different widths. I believe 4", 6", 9", 12",. Although, most stores will likely only have a few options. I like the 9" roll for signage, since you could cut out several 9"x12" pieces from a single roll. Or, if you were making signs from OSB, you could roll strips to cover the surface, or cut out letters to make board signs or whatever. Anyway, next take spray rubber and coat the writing side (not the sticky side) of the protecto wrap with rubber spray. I used the black plasti dip, which I know will work. And it almost fuses to the surface of the tape, like magic, making a nice canvas after a couple coats. Once dried (about 4 hours) can be spray painted extremely easily, or painted with acrylics and I'm sure other stuff would work. Or it could be just left black if that's what you want. I know they make other colors, but the lettering from the protecto wrap did kinda bleed through the white plasti dip, when I tried that and it looked kinda funky and not nearly as trustworthy of a canvas.
I made some small stickers, way back when, but kinda ran out of the supplies I had, and I also have no way of really getting around to actually engage in real world things because clown world likes ruining people's lives for no fucking reason. Best of luck