For an undersuit I would recommend not using Spandex for the reasons mentioned above. Instead try a fabric manufactured by Polartec. Yep, that's the company that makes the best fleece in the world. The fabric is called Power Dry. It is a lightweight breathable base layer designed to wick moisture away from your skin. Keeping you cooler and more comfortable. As a bonus, the fabric is also available with Odor Resistant Technology, so your undersuit won't smell like something (or someone) just died in it. On the issue of padding, I think mesh pockets that individual pads can be inserted in is the best way to go. Removable padding means you can wash the undersuit (back to the smell bit). A combination of open and closed cell foam pads should work well. Just remember to break the padding into segments, as closed cell foam doesn't really do compound curves in the most comfortable manner. Good Luck. The source for Polartec fabrics for the home sewer is Malden Mills Direct Textiles . Beware of imitators.