I decided my ultimaker could be improved by being able to print ABS. So naturally this meant making a heated bed, but recently I went one step further by enclosing the whole thing in a bunch of oven bags, in so making a sort of warm chamber. The chamber temp is not controlled and there's no fan in it, the bags are there just to keep the whole thing warm and help prevent warping.
So underneath that 3mm aluminum sheet there are four 30 ohm power resistors JB welded to the bottom. A 100K thermistor in the middle of the bed provides the temperature readout. The wool towel clipped around the bottom serves as insulation. At the bottom of my UM frame I put a small perfboard with the solid state relays that power the resistors. 120VAC through 120ohms of resistance yields about 1 amp and 120 watts of heating power. That is a bit on the low side so it takes about 20 minutes for the bed to heat up. The slow heating was part of my motivation to make the chamber.
The chamber itself is made of cut up oven bags, taped along outside the walls and on top. The cross bars on the xy gantry keep the top bag from falling down and fouling everything up, which is nice. Also after its been printing for a while hot air travels up and inflates the top bag a bit which further helps prevent it from snagging on stuff.
I had to take off all the PLA stress-bearing parts, including my bowden clamp which was sitting on top of the insulating bag. I imagine i'll have to take off my belt tensioners before too long too, but they don't have to bear as much force as the bowden clamp did.
Still trying to work out the bugs in the filament feed system (my ABS is just slightly too fat but it seems to work most of the time) so I haven't done any enormous prints or tried to compare warping vs non chambered setups. That is what i want to do eventually though.