The Jamie Baker? The FRA RCC is probably your best bet. The showers there are dimly lit, but fairly comfortable. The F lounge area of the RCC is intimate, but last time I was in there had a decent array of hot finger foods, wine, champagne, etc.

It will be much more comfortable than any of the LH lounges (all LH Business Lounges have showers, if I remember correctly).

Shower at RCC in FRA?
http://www.flyertalk.com/forum/Forum50/HTML/017105.html

FRA: Customs/ LH /Arrivals by United
http://www.flyertalk.com/forum/Forum50/HTML/011835.html