My apologies for the delay in answering this request.
The most widespread customized gear is definitely helmets and I have a tag for that: #helmet art.
I don’t tag any other specifically customized gear however. I know there are some customized flak vests in the archives, but would require digging through the tag #flak vest. One photo of a US Army sniper features his camouflaged rifle. Another photo displays an M14 with a modified magazine. There are a couple other modified weapons if you “/search/modified” on my blog.