Just attach this to your walls and be done with it:
Polystyrene Foam Sheeting (more commonly known as Styrofoam):
This is excellent for harsh environment growrooms (your attic for example), provided you have a good ventilation system and a way to keep the temperatures from rising too high (an a/c unit or similar) as it is an excellent insulator.
It is also a great material for use in a temporary setup or for use as a "travelling reflector" on a light mover, where weight is a concern. It is approximately 75-85% light reflective so it is comparable to using a flat white paint. Foam will not create hot spots. Rigid foam can be purchased in sheets, and can be used as a free standing wall or can be taped, glued or nailed to the wall, the last generally being the most successful method.
If you have to make a frame of some sort consider using PVC. It is light and cheap and with so many fittings and couplers etc. with just a hacksaw and the stickum you can build anything in any shape cheap and easy and with very little mess.