Heating the Bed for OpenMultibot

Please give your suggestions for heating a bed 300 mm width and 1200 mm length:
OpenMultibot Bed

Lower cost and simplicity is the mission.
For now the choice is using 4 BCP regular plates $11 for each attached to the bottom of each oven glass plate 300x300 mm.
Also preferred is a solution in which the heaters are fixed to the frame and the glass plates can be easy removed for cleaning so they are not physically connected to the heating elements.

Maybe some one heating element with a fan circulating the hot air inside the frame which need then to have a closed bottom.

Seems difficult. Not bonded, so lower thermal conductivity. You can’t press something against the glass if you want to keep it flat. Hot air won’t have any more thermal conductivity than the free air above the plate, so thermodynamically you will have trouble maintaining temperature without making the air in the box under the plates quite hot. Are you thinking of taping down top-side thermistors in use?

Personally, if I were doing this and required to use glass rather than cast aluminum for the bed surface, and cast aluminum were not available, I think I would bond to heaters (with thin aluminum sheet as a heat spreader if necessary for the heater type), with connectors to disconnect power and thermistor for washing off the top surface, and use high-quality electric connections so they didn’t catch fire.

Thinking wayyy outside the box and going beyond what I’ve heard of anyone doing before, since it sounds like you’ve already thought of that, have you thought about a water bath in that frame with the water touching the plates when it is at target temperature? You could use tape on the bottom sides of the seams between the plates to keep water from getting to the print, perhaps. Probably you would want to seal the top down to the box too. Then a pressure relief off to the side; a filler tube that goes above the surface level, combined with a dump valve to let water out before removing the plates? It’s a crazy idea and I doubt it could work, but maybe it triggers someone else’s ideas that are better?

To reduce the span of research I shall print with PETG: I need about 50°C on the bed for adhesion at this span of 1200 mm and what is most important that I need the part to take off when the glass is at room temp (this has worked always for me for 300x300). Therefore I made it out of 4 parts 300 mm each because I know they do not crack of sudden cooling.

I found some sticky heating foils in my scrap do not recall where I bought them years ago. In the corner where my finger is written 12V and I am still heating it barely getting 29°C.

Any insight on this matter? Looks light enough to be used for the glass of Multibot.

I remind: the value in heating in this case is not the adhesion but that the part pops up when the glass cools down, so I need about 50°C on the surface.