Very innovative Jamie to use the translucent box for a "tent". After way too many years as a professional photographer that has never occured to me to use a plastic extrusion in that manner. You can be very creative with that. For instance.....
-if you want to limit your lighting on one side or the other for effect, simply pull the light back, or "feather it" by turning it one direction or the other from your subject.
-you could for effect, use black paper to block one side or the other to limit light for dramatic effect
-use colored gels for effect
A couple suggestions....just IMHO...
As far as the image looking overexposed, I think it might be because your lights are a little close. It is lighting the bottom of the pen. Maybe try bigger bulbs, back them away from the box. This will allow "hot spots" to be limited. I might try a black gobo (photo talk for go between) or the cardboard you are using in front---it may not be as good, as you may be getting a light kick back that is adding to the lighting. YOu may though, get a better reflection for some pens. Certainly something to try. Exposures are ok if they are long too, so less light is merely compensated for in longer exposure...just make sure to use a tripod and your self timer.
The backround being yellow is merely a one click correction in your software---great photographers are partially created in the manipulative software---although none want to admit it. There are some good free ones. You do not need a 700.00 photoshop for simple things. Try some different ones, decide on one, and PRACTICE with it....
You have a great start and a very innovative idea---many on this site that do not have the money or desire to buy a tent could use this idea....
Moke