Get a DAC for the arduino. Don't waste your time trying to filter the PWM.
When the correct tool is so cheap.
Can you afford $5 ?
You still need an OP AMP Non-Inverting , gain of 2, amplifier, running off +12V dc
See attached datasheet of op amp.
FYI- R2 & Rf should be SAME value because an op amp non-inverting amp gain= [(Rf/R2) +1].
LT1215.pdf (339 KB)