FWIW, I have a really weird plan for my heated bed. I haven't built my Wilson II yet, I'll be waiting for parts to ship for several more weeks, but I figured I'd share my plan. I decided to build a normal 200x200 bed using a piece of 1/4" cast aluminum tooling plate. From what I've read, these are essentially perfectly flat, and when paired with a silicone heat pad will heat up quickly and evenly. I decided to go with only 200x200 because this stuff is heavy - at this size it will be ~1.8 pounds, and if it was the full 200x300 it would be ~2.5 pounds!
I'm going to use a material called aluminum composite panel (brand name Dibond) for the y-carriage (1/8"). This is made from two sheets of aluminum bonded to a core of plastic, which supposedly is pretty rigid but also lightweight. I've come across a few posts on other forums buried in the depths of Google that talk about using Dibond for y-carriages. I think I'll be able to cut this using a jigsaw and a metal cutting blade.
Anywho, my plan is to use 3-point leveling with this setup. I don't have a sketch yet for my carriage plate, but I anticipate putting 2 leveling points at the back and the 3rd in the front center.
The reason I decided I could get away with a smaller build plate is that I don't anticipate doing huge prints in ABS for fear of warping. If I want to try something that would use the entire 200x300 bed, I will put a piece of glass that size on top of the cast aluminum plate and hope that it heats evenly enough for PLA.
I'd appreciate any comments from anyone with suggestions or to point out major pitfalls in my design plan. YJL, I hope this helps, or gives you some new ideas!