How do you heat a large glass print bed?
It will be very difficult to heat such a large bed, simply because of the enormous power required. Thomas Sanladerer recommends at least 0.6&nbsp;W/cm² but notes 0.4&nbsp;W/cm² also works (but takes "forever" to reach the target temperature).

For a 30"x30" bed, 0.6&nbsp;W/cm² would come out to 3.5&nbsp;kW. At 110&nbsp;V that would require 32&nbsp;A and at 220&nbsp;V, 16&nbsp;A. These are extremely large currents, perhaps more than you can draw from a single circuit: both in the EU and US sockets tend to be fused at around 15-20&nbsp;A (the standard EU Schuko plug itself is only rated for 16&nbsp;A).

You will be forced to go with a lower amperage, for 0.5&nbsp;W/cm² you "only" need 27&nbsp;A@110&nbsp;V or 14&nbsp;A@220&nbsp;V. 0.4&nbsp;W/cm² is 21&nbsp;A and 11&nbsp;A respectively

As such, if you are in the US, then it will be impossible to heat such a bed from a standard wall outlet. In the EU it might just about be possible, but make sure that the wiring in your house is in good state and capable to carry the current required (and note that running electrical equipment at its maximum rating for an extended period of time is never a good idea).

If you are in the US, you should definitely look into getting 3-phase power installed. If you are in the EU, you might also consider this as an option. It is able to deliver more power, but you'd need a special type of heater.

There are some suppliers that make custom silicone heater mats. If you can get one in this size that would be a good option, though it would be considerably more expensive than your \$5 piece of glass.

In any case you should not attempt a DIY solution because this is extremely dangerous. The currents you are dealing with should not be underestimated. People have already set their printers on fire due to using high currents at 12/24&nbsp;V; you do not want to make a similar mistake using even higher currents at 110&nbsp;V.