D-Bot coreXY heated bed support

I'm well on my way to building a D-bot and have got to the stage of fitting a 200 x 300 mm heated bed.
It looks as if the design has it sitting on 4 springs with level adjustment bolts to attach to the bed supports.
The heater is very flimsy and is sagging in the middle of the long sides and with a glass plate on top I would have lost contact with the heat bed at the centres.

I am wondering if another support should be fitted along the centre, or is there another better way of doing things

I was going to use cork as an insulator but that is also flimsy, should I use a piece of plywood?

Thanks for any help

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appjaws - Core XYUV Duet Ethernet Duex5
firmware 3.1.1 Web Interface 3.1.1
Ormerod 1-converted to laser engraver, Duet wifi
OpenSCAD version 2020.07
slic3r-1.3.0, Simplify3D 4.1.2, Cura-4.4.1

I have another question.
What should I use to glue cork sheet to the underside of the heat bed?

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appjaws - Core XYUV Duet Ethernet Duex5
firmware 3.1.1 Web Interface 3.1.1
Ormerod 1-converted to laser engraver, Duet wifi
OpenSCAD version 2020.07
slic3r-1.3.0, Simplify3D 4.1.2, Cura-4.4.1

Glue the heater to the bed plate if it's sagging. Use high temperature silicone.

Use 3 point leveling instead of 4 point leveling if you want to actually level the bed and keep it flat.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]