I bought a small green a couple of years ago for $40 about the same size as yours. In my case I do not use any artificial lighting but I do have it setup on the south side against a large window. I found this method to work just as well and doesn't cost me any electricity.
The drawings were produced using 3D modeling software called sketchup. Sketchup was purchased and is currently distributed by Google. I use the free version which can be found on the google homepage under the "more" tab followed by the "even more" tab.
It is increadibly powerful software that is fairly user friendly. I have found it is a good idea to "create" a new design as a scale 3d model in this program vs building it in the real world. Saves me a lot of wood and frustration. Be sure to check out some of the tutorials, there are plenty on youtube. It can be really tough to draw in 3D unless you understand the color coding system and the inference point system in use.
The images are simply views of the virtual model that have been exported as 2D .jpg files.
It is an amazing piece of software. Very empowering.
Search youtube for "sketchy physics" or "sketchup animation" to get an idea of its full capabilaties. It is pretty much only limited by your hardware. I only use it in a very simple way.
Other people are using it for art and animation, for engineering, for landscape design, for decorating, you name it.
The free version has most of the features of the expensive pro version but with the pro you can export to CAD, print 3d models and use the data in other useful ways.
Some of the features that I think are really cool;
You can add dimension lines to a pic and create a 3D model out of a photograph.
You can import textures and colors from pics and other sources.
With the sketchy physics add on you can make wheels that turn, and gears that move and such.
You can make a video walkthrough or flyover of a model.
The ability to make accurate 3D topography. (still figuring this one out, but I think it could be a real asset for a permaculture designer trying to explain water management.)
access to the 3D warehouse where you can find millions of other people's drawings to import into your design. For example, the florescent light fixtures in my drawing were imported from the warehouse and took me all of 30 seconds to add to the model.
Sometimes I wonder if I can trust Google with my info, as they are starting to look more like big brother every day. But things like this I feel reveal a philosophy of empowering people vs. subjugating them. Makes it easier to live with the fact that they already know everything about me.