There are heavier armours than the 5D of a hauberk... but they are not for PC's. Specifically, the armor of a cold drake is 6d. (Troll-scale is only 3D to 4D, for comparison.)
The best armor I can find described in the novels is "brazen plates" (Ch 4 of The Two Towers):
Sam, eager to see more, went now and joined the guards. He scrambled a little way up into one of the larger of the bay-trees. For a moment he caught a glimpse of swarthy men in red running down the slope some way off with green-clad warriors leaping after them, hewing them down as they fled. Arrows were thick in the air. Then suddenly straight over the rim of their sheltering bank, a man fell, crashing through the slender trees, nearly on top of them. He came to rest in the fern a few feet away, face downward, green arrow-feathers sticking from his neck below a golden collar. His scarlet robes were tattered, his corslet of overlapping brazen plates was rent and hewn, his black plaits of hair braided with gold were drenched with blood.
Sounds like bronze plate to me. Probably of the Wiesby or Lamellar modes.
It's easily enough extrapolated to add this as "field plate" (24 Enc, 6d). Doing so means you'll be weary right quick, and that's fine. It's a trade off - losing a significant portion of your chances to hit, in exchange for being almost immune to being wounded.
Lesser forms of plate may be worth adding, as well, but should overlap mail, not be increased levels. Those would be breast & back, and partial plate... and they should, in all likelihood, be Enc 8, Protection 2D and Enc 16, protection 4D armors.