The purpose of this topic is actually to provide people with a true icon template, rather than asking for help about one.
So I followed multiple tutorials for creating icons in GIMP, and I successfully did so. While running the game, I somehow noticed that there was a strange dark outline around my custom icons that seemed off. My attention to detail was soon proven to be correct. The icon "template" I downloaded that was provided in a tutorial topic was not good quality, causing it to differ from default icons.
I then created a custom icon template, forged from the default wand tool icon, since it provided the most area of the background. I successfully managed to create a true icon template that I have provided in this topic for anyone who was having the same problem, or anyone who just didn't want to go through the hassle of cutting and pasting and filling in their own template.
The entire image is meant to be 96x96, and the gray circle is meant to be 82x82. Anywhere within the 82x82 space not taken up by gray pixels that are part of the circle is meant to be completely black (0, 0, 0) but completely transparent as well (mostly in each corner). The surrounding area is meant to be completely white (255, 255, 255) and completely transparent. I messed up a bit and some places in the image where it should be completely black transparent are completely white transparent instead. You'll see what I mean if you import the png into GIMP. Hopefully, that doesn't cause any issues.
If you want to compare each individual pixel in the template to a default icon to check my work, be my guest. Please notify me of any problems or concerns. Hopefully, I helped some of you out who were planning on making icons for weapons or items.
Update 3/31/20: I don't recommend using my template. It has a white outline around it. I haven't been able to fix it yet. Try using the ones other people posted down below.