A heated bed should help
Hairspray & glass bed is the best.
Stick glue works. So does hairspray.
Slower cooling with heated bed or oven. If you have the base metal plate, you could always add threaded mounting points for a standard borosilicate glass top, leaving a small gaps to level bed with screws and bushings, the Printbot does have the level sensor to adjust for the added height. You could then add one of the adhesive heat pads to the underside of glass and then, if you can’t connect to internal power supply then use exterior one and method to control power and readout of temperature. Also you can enclose the build area to keep warm and allow for slower cooling which too fast of a cool rate is what causes the shrinkage and thus warpage. Then using an adhesive spread or glue stick will help too.
I print with nozzle 215 bed 75 no cooling and just on glass cleaned with aceton. Bed levelling with a normal a4 paper. Every part is sticking 100 percent.
Philip Ramirez is absolutely right. PLA warps only in extreme conditions. This not the case.
Just solved this on my Prusa i3 with PLA. Turned the heated bed down to 30 C (~off), cranked the speed down to 30 mm/sec, turned the fan (on extruded filament) to 100%, infill set to 25% (some other minor changes too but these were the biggest) - using green painter’s tape.
Extruder temp is 220 C, hot enough to bond to cool part, fan / speed combined set to give time for extruded (and bonded) filament to cool BEFORE the next layer is put down - this keeps each new layer in its
proper position, even if the previous layer has shrunken a little in cooling - thus shrinkage is corrected - layer by layer - instead of allowed to build up - as it does if multiple hot layers are laid down and gradually allowed to cool - from the top (air) on down (bed and/or 100% infill thermal mass). Good luck and
have fun !
Just spray the bed with isopropyl alcohol and wipe down right before you print. It will stick like crazy