DH's team kit from L Garneau has similar problem. Pro actively, I would suggest a small backround print (grey on the black) to provide distraction, or words printed over the entire area. Since his kit is already created, as is yours, the only solution is a base layer long enough to pull down over the butt crack area under the shorts. His lightweight icebreaker or ibex baselayers are long and stretchy enough to accomplish this. Tokie