Thanks guys for the compliments, I appreciate that very much! Sorry that I did not take step by step pics at the time. I was inspired by a guy in the states who had done this to his Neca. ( see link) hope this helps.
It has been a while back when I did this so I will try to describe the mod the best I can; Important to not try to take the figure apart when cold, you have to put in hot water for a while to soften the plastic. Still be careful but it will take some muscle to take some joints apart. Cape removal before you do a Batsubmerge Then warm it up ( 60 -65°C = 149°F) then take of his head which came of not to hard as I remember, just be careful not to angle the joint to much for risk of breaking the joint. The cloves were relatively easy but the boots were clued with superclue, so you may need some tools to prie? pry? ( sorry ) them loose. If you damage the inserts is no big problem because they will be hidden when mounted again.
When boots are removed ( try to keep figure constantly warm in hot water!) then I took the belt of over the hips down ( plastic is warm so it stretches) then I noticed that his trunks were glued in the middle with a little square notch of the front and back, so again keep the plastic warm and pry the trunk loos from the "pelvis" part. Also take trunk down and keep plastic hot ( sounds weird)
Now you have a pretty workable piece.
You can try to keep it this way so without taking lower and upper body further apart.
The textile I used was pretty stretchy which is a necessaty for a nice tight fit.
Make templates of the upper body; first I did the arms apart but that was way to hard and ugly, so I simply made a template by taking a piece of Template paper? ( used by sewers/tailors) and make a template like drawing , this way you get a simpler and easier to make suit.
I used two layers of fabric because with one layer the belly joint was still to Obvious. If there is a difference in the fabric upper or under make sure which side you want to see, and sew the suit inside out together. I know it will take some hours of sweat and maybe some bad words but eventualy I hope you guys or wives/ girlfriends will manage .
The legs are easy, also first make a template on paper and again double layer or if fabric is thick just one. and sew inside out the two seperate tights. Some extra pics to hopefully make things more clear. Good luck!