What is the exact diameter tubing i need to make my own . I already know it needs to be madrel bent (covered) . I do not have calipers to measure the TBI OD" for the Tube .

2.5 - 3". Maybe 3.5".

You would spend more trying to get it right than just buying one. The length has to be correct, the placement of the bends has to be correct, and the diameter has to be correct. If any one of these are wrong then you will probably loose HP instead of any gains. A AEM cold air is only $200 and will give you the best gains out of any other intake, around 3-5 HP.