first off attics can be a poor spot for growing depending alot on your climate they can easily be excessivly hot in the summer and to cold in the winter and im sure the 400 hps without any ventilation to cool it can easily raise the temperature by 10 degrees so if at all possible a better location should be considered.
tape, cardboard, foam doesnt really sound like a very good idea the floor needs to be strong plants soil and pots just watered can be quite heavy 3/8 plywood or thicker would be adequete for a floor to support you and the plants.
while you can use other materials to make temporary walls even cardboard, foam, plastic sheeting could be used it does need to be secure so no tape holding it up staples, nails or screws with large washers should be used so there is no chance of it falling.
obviously anything that gets hot should not be very close to anything flamable your ballast and hps light is your main concerns your light should be secured with heavy enough chain to support its weight, and you may need to keep the ballast off the floor also depending on how hot the case gets most of this stuff is just common sense but it can go a long way in preventing a fire if you do not feel confident in wiring the room up you should probably find someone to do it for you or atleast find someone with experience in home wiring to help you, in fact that should be the first priority before a room is even started.