You can use a N-VFET (cheap, easy to get) and a charge pump and level
converter to shift the 12V to something the high side mounted FET can use.
I did it that way once and it works well. The charge pump used an existing
square wave signal and a number of CMOS gates in parallel. The level
shifter was a section of a LM324 opamp wired as DC amp with gain x10 or
so. My switching frequency was 1000Hz. This was for a motor. The noise was
a welcome addition (warning noise).

Peter

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