*cough*
http://wiki.teamfortress.com/wiki/Die_Regime-Panzerung
and *cough*
http://wiki.teamfortress.com/wiki/Der_Maschinensoldaten-Helm

Stats could be:
+34% Bullet resistance
+20% Blast resistance
+10% Melee resistance
-10% Movement speed on wearer
+25% Fire vulnrability
-25 Metal capacity
-Replaces pistol/ wrangler

Or alternately:
+50 health on wearer
+10% Bullet resistence
+Immunity to Mad Milk and Jarate
+Crits recieved reduced to mini-crits
-Crits dealt reduced to mini-crits on wearer
-Cannot be buffed by banners
-50 Metal capacity
-10% Movement speed
+10% Fire vulnrability
-Replaces pistol/ wrangler

The first idea is based upon the defensive engineer, allowing them to stay around their sentry longer when under attack, however for this they sacrifice manurveraility (making backstabs more likely), make pyros more effective as sentry busters, and damage their medium range firepower or ability to control their sentry, making it harder to defend corners.

The second idea is based upon the concept of dempening down buffs and debuffs placed upon the user, at the cost of metal capacity and a slight vulnrability to fire. This would make it good for Mini-sentry users as they would be somewhat more durable in combat.