which framing materials are ok for a very sturdy box- that is able to withstand regular movement and 'rough handling'? metal or aluminum? the box i have in mind will be about 4ft by 4ft by 4ft. i.e. 4ft cube. Also, in view of the size, i plan to have the box move on wheels or suitable castors. the core material for all the sides, bottom and lid will be extruded polystyrene([pink or blue board)- 3 inch thick. will this thickness be adequate in view of the size?