if you want it permanent and to go outside the jumpsuit, goop works great. I used it as a temporary attachment method a year ago and it turned out to be permanent! So if you don't want the elastic to go through the jumpsuit and plan on sliding the leg pieces on, then i highly recommend gluing the velcro straight to the leg armor with goop.
On both my Fett suits, I have the gaunt undersides attached with woven "non-rolling" elastic straps from Walmart. The white or black kind. That way, they stretch around my arms when I attach the front velcro. The best method I've found for them, is Zap CA. I attach it and use wooden clothes pins or weights to keep them firm. The next day, they're pretty stout. I use a lot, so I leave it to dry.
For armor pieces, like Trooper thighs, ab plates, etc., I use super strength velcro on the armor, and elastic garbage can straps (at Lowes) that have the velcro ends. There are two going around me , and two "suspenders" for my ab plate.