Look up the power requirements of the projector, voltage and current or watts.
What you need is an inverter, and those are much too complicated to build yourself, but they are not very expensive, depending on the size (see first paragraph).
And inverters are not made to operate on 9 volts, they need 12 volts or higher. You connect the inverter to the 12 volt battery (and it probably has to be a big one) and it has a 120 VAC (or 240 if you live in Europe) into which you plug the projector.
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