I will try my customized silicone heater pad.  Incl.

I will try my customized silicone heater pad. Incl. shipping and cable
insulation only 34 Euros!
Be careful i have spezified or customized all options!
Designed for 300x300 build-plate.

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It will be interesting to hear how long it takes to heat up. Mine is 330x330 on a 350x350x6mm alu bed. it has 350W on 24V and it takes about 20 min to reach 90°C. Max temp is 105° (NTC temp). The alternative setup for faster heating up is a 240V AC silicon heater and a Zero cross SLR.

Not received yet!Alu bed also 300x300x6mm. (0.54dm^3)
I want to insulate the bottom and outer area of
the bed with cotton.The time you tell makes me afraid …
Your volume is 0,735 dm^3 or nearly 50% higher and the
squarematrix is also greater, i’m confident (!?)

6mm is thick and it takes a while to heat but the temp also stays stable due to the stored energy in the bigger volume. It also takes a while to cool down and you need to be careful not to burn your fingers when you remove the part while the bed is still hot…
Another advantage of 6mm is that it does not bend when heated up. Depending on the size of the bed, 4 or 5mm might be enough though.

I had no choice, only 6mm strenght is be milled out of
aluminium casting-plate tension-arm.

Solution for heating up (joke):