I have a diy I use on my 45gl. If you have somewhere under the tank you can store the system you can just use a longer hose to get the co2 into the tank. Seems like a good Idea for someone who wants a diy co2 system but doesn't want to build all of the components or just wants something with a cleaner look.
Another cheap non diy you may want to look into is the small fluval co2 systems. Its not the greatest but for the price I it works good enough. Its $25 on amazon I think, pretty expensive in stores. I also ordered a better diffuser with it for around 10 bucks. They also charge a ton for the 20g co2 refills, its around $15 for 3. However you can order 16g THREADED co2 cartridges off ebay for around $1-$2 each or buy a converter to use a larger refillable paintball gun co2 container. I'm not sure how well it would work for a 20gl, but it works great for my 10. Again this is not the greatest co2 system, but I really like it. I didn't want another diy with my nano and I wasn't ready to spend the money on a good system. They also have a slightly larger system for around $50, but Im not sure there is a way around buying their co2, besides possibly doing the larger refillable paintball gun tank adapter.