Since the shape of the new frame does not lend its self to the lines of a human skeleton I need to add some non-structural parts so that the costume hangs right.
To get a decent bend I applied heat, first with a small space heater, then with a heat gun.*
Once it was hot to the touch (and my work space had a very vague odor of plastic) I used a ratcheting tie-down strap to apply compressive force on one side (in other words I put one hook on each end and ratcheted away). I only used as much force as I was able to using one hand.
Over the next few hours I moved the heater and heat gun around to various places and tightened the tie-down strap enough to get a nice gentle, spine-like bend.
I used the same method to un-bend the base of the spine (the lumbar curve).
After consulting my friend Deb (a licensed massage therapist, who has forgotten more about anatomy than I will ever know) I used more of the cheap irrigation pipe to make the Spinous Process and the Transverse Process.
*IMPORTANT SAFETY TIP: Do not, under ANY circumstances, use an open flame to heat plastic pipes. ABS, in particular, catches on fire quite readily. Nothing will ruin your day faster than a ten foot chunk of blazing plastic in your workspace. Such a dramatic event didn't happen to me, but I did set a 1 foot section on fire a few years back (I was trying to smooth out a badly drilled hole by using a MAPP gas torch) ...dumb.