Traditional pinstripes to fill always look cool, and they can be used around existing art or even stand on their own. They could serve to tie your whole piece together, if done correctly.
Without seeing pics of what you have I'd suggest you go with something similar to what has already been suggested... the stars and maybe some tradiational dot fill could work. Important thing will be going with simple designs and colors to make your current work "pop" instead of just cluttering you arm. Post up pics and we'll see what we can come up with.
__________________ WELDON MALBROUGH
SSG, USA
Fort Bragg, NC
Traditional pinstripe. I'd stay away from company logos, etc. I'm pretty old school traditional when it comes to my half sleeves (I have one dragon koi fish, and on the other arm, a phoenix, both traditional Japanese style). If it were me doing something like your tool theme, I think a pinstripe background would look good.