I don't know any suppliers of 300x300mm heater PCBs. Why not use one or more undersize silicone heaters and an aluminium heat spreader plate? I found some inexpensive 12V 100W 235x100mm silicone heaters on eBay. Two of those should do the trick. You could drive one from your Rumba board, and use an external mosfet to drive the other, to avoid overloading the Rumba.
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