Personally, I wear black compression cloths, and I have a foam/faux leather ab wrap piece.
Links to how some folks did their under suits:
[URL unfurl="true"]https://www.405th.com/forums/threads/rangers-halo-reach-build.50240/post-791402[/URL]
[URL unfurl="true"]https://www.405th.com/forums/threads/pipninjas-reach-spartan-undersuit.54119/[/URL]
[URL unfurl="true"]https://www.405th.com/forums/threads/reach-jun-build-foam-fibreglass-custom-undersuit.45382/[/URL]
[URL unfurl="true"]https://www.405th.com/forums/threads/detailed-reach-armor-undersuit.49218/post-768412[/URL]
[URL unfurl="true"]https://www.405th.com/forums/threads/n8tebbs-mk-7-build.54089/post-827492[/URL]
[URL unfurl="true"]https://www.405th.com/forums/threads/jerverants-mkvii-%C2%B7-anubis.52995/post-824970[/URL]
[URL unfurl="true"]https://www.405th.com/forums/threads/monker%E2%80%99s-mk-7-build.52843/[/URL]