I made an aluminium bed caddy for the Printrbot Plus (V2) out of 5mm plate due to the wood on my printer being very warped out of the box.
I must give credit to this thing (http://www.thingiverse.com/thing:91504) for inspiration.
It sits over the stock bed to utilise the levelling features. It takes a little longer to heat up completely but once hot, it stays hot.
I'm considering using an adhesive to bond the heating element to the aluminium plate to further reduce the warp in the stock glass. I'll post an update if or when I do this.
(Update) I ended up cutting another piece of plate 230mm x 220mm and fitting it over the heated bed. It works extremely well! If you're having trouble with lining up your please be aware, the heated bed isn't square! (mine isn't anway). There is about 3-5mm across the diagonal in mine so ensure your holes are in the correct position.
One more thing, please calibrate while heated in a stable environment. My printer is in my garage and the ambient heat varies 5-10 degrees C during the day which effects the bed quiet a lot. Calibrate before every print is my advice.
I added approximately 3mm either side of the heating bed to the plate and measured to the back edge of the slot where the wires for the element go. I then cut the 5mm aluminium plate with a guillotine to specs. I cut the slot at the back with a hacksaw. I then used a grinder to smooth off the edges.
Once cut and cleaned up, I placed the element over the plate, I centred the element over the sides and front end and marked the position of the holes to be drilled. I used a 3mm drill bit and a cordless drill.
Once holes were drilled, I lined the holes up and inserted 20mm M3 screws into the holes (NOTE: you may have to use a small washer on the head end of your screw\bolt to ensure clearance of the travel bed, depending on how level your bot is) and tightened just past finger tight.