I looked into making a diy uv filter. IMO it's not worth the hassle. I love diy and have spent more on some projects than just buying a premade product.
With the uv light safety was the biggest hold up. It can't leak light or it can be bad on you and/or your fish. Plastic and glass choices are limited because of uvc's properties. Parts are not difficult to find but are pricey. If you do go diy I would love to see what you come up with, I love seeing peoples solutions to problems.
My idea was a clear tube surrounded by uv lights. A aquarts tube alone is pretty pricey.
I ended up going to petsmart and buying a greenkilling machine for 50ish. Ran it for 2 weeks, water was clear after 5 days. Now it's in the Cubard. If you want a inline light I've seen them with pond supplies for around 80-100. Simply splitting the return flow and using a ball valve would be sufficent to plumb it.