After learning that the sauce was over $5 for a bottle of 170mL, I decided to search the internets for some recipes.
So after a bit of averaging all recipes I found, here it is.
-1/2ish cup of dark soy sauce (you can use light soy sauce for lower sodium)
-1/2ish cup of mirin (sweet rice wine)
-1/4ish cup of sugar (granulated sugar is fine)
On SLOW heat, warm up the soy sauce and mirin. Once you have a warmed up pan of the two, pour in your sugar. Let it simmer...don't forget to keep stirring because I find that the sugar might stick to the bottom of the pan. When the sugar has melted all the way, you're good to go! Let it sit and cool before using.
*optional* You may want to add in a little bit of cornstarch to thicken it. I recommend first taking some of the soy+mirin+sugar sauce away from the heat in a bowl, and mixing in the cornstarch to that. Make sure all the cornstarch is dissolved before pouring the contents of the bowl back into the pan.
Usage: Unagi BBQ of course. You can also put this sauce on other seafood too that you put on the BBQ.
Hope you guys enjoy!