I dont think the Mishimoto powersteering reseviour is junk, it just wont work with the Z (I've tried... several times!). I know Simota1 had his OEM powersteering reserviour wrapped in carbon fiber, which is what I will do this winter as I dont think there are any direct replacement options for the 370z yet. As far as coolant overflow tanks, I had one custom made by LCR Autowerks (a forum Vendor/Sponsor), I also think ARC re-released their coolant reservior/ catch can combo which is really nice. I saw one on JapanParts.com awhile back. I also think forum member Navyboy916 bought one recently. Here are pics of the two options:
LCR Autowerks Option:
ARC Option: