Springy Corner Bracket
Uses custom spring. Springs can be ordered at http://www.ultibots.com/springy-corner-brackets-x4.

Version 2 directly couples the heat bed to the corner bracket using Small 3/4" (19mm) Binder Clips.

The springs are .600" od x .053" music wire x .500" long, 3.6 coils, closed and ground with a plain finish. The load is 12.4 lbs each.

If you are going to heat the bed > 100C, print with ABS. PLA will deform with high heat.