I'll tell ya, I built my own tried out of window screen... I did the math would of been cheaper to build it myself but I went ahead and added some extra things that I thought of along the way... Between that and the cost of materials and the time it took (not too long but you need to set aside some time to just get it done, I made the mistake of cutting some pieces and coming back to it days later) If you do that, then you risk the chance of forgetting what pieces you cut and what pieces you need. I would say its easiest to buy a reptibreeze, I thought everything was going to fit together like a glove with the DIY but it took some figuring out for some parts, and if you don't add a bottom to it and screw in the top and bottom then it will be quite flimsy and sway to one way or the other.
Honestly, I built my own and enjoyed it for a couple months, but I went ahead and bought a reptibreeze anyways. Going to see which one I like better and probably make a separate thread on the pros and cons of each.