For all the times that you made fake HB merch, where did you get the BG? you've really inspired me!
For the pirate standee background, traced the official standee background and colored it, so I could have a clean version of it. I would share the blank version, but I only did the parts that were visible, so I dont actually have the whole thing.
for the valentines posters, I recreated the BGs after looking at the official merch. I do have a blank version of one of them! I shared it before but here it is again. all you gotta do is add the black hearts (those are placed uniquely around each character rather than being a set part of the background).
As for this shark robot bg that I used with my fake pride pins, thats just from this picture of an official pin. The pin is small enough that I could just put my drawing on top and completely cover it.
They dont use this background on shark robot anymore and instead just have transparent backgrounds, but its still a pretty cool pattern.
so, for the most part, the answer is to just try to match the official background as best you can. Trace and color drop to get it as close as possible. you might have to look at several different pieces in the same merch lineup in order to see all the background, since different characters will cover different parts of it.
fake pins and keychains are the easiest as far as this goes, since they don't have backgrounds. all you need for pins is a shadow (duplicate the drawing, move it under the drawing and down, color it black, gaussian blur it, lower opacity until it looks natural)