Chris,
I built my light box out of foam board from office works, hot glue, spit and solder, but I saw one of
Exfso's boxes at pmrid's and it's built way better than anything I could ever knock up.
When shooting flats, I normally aim for the center of the hystogram bell curve to be around 20,000. As the maximum is 64K, 20K is close to 1/3. Some of us with slow shutters have problems getting the light dim enough to achieve this as we need to expose for 4 seconds at least to avoid shutter artifacts in the flats. Introduce mono cameras and binning and the problem gets worse. But with a one shot colour it shouldn't be an issue.
Here's a flat from the other night plus the image I used it on, rotated 90degrees CCW, I've been chasing eggy stars
The black bit to the left of the flat is the prism from the off axis guider sticking too far into the light path and yes it needs a clean but they flatten out the image a lot
Also, if you are going to get an LP filter, get an Hutech IDAS filter not a CLS. The CLS will throw a colour cast over the image which I found near impossible to correct at the time, the IDAS does not do that but it costs more.