In your case, I think it would be beneficial to work on purely rhythmic exercises that only involve percussive sounds and body movements, like clapping, walking, percussion instruments, etc. In this way you can devote your focus to polyrhythms, and not have to worry about things like harmony, fingering, and other such challenges. Once you have established a polyrhythm with clapping, then you can supplement by studying a piano piece that features that polyrhythm.
Start with the most common 2 against 3, as well as 3 against 4. Go through variations of being able to clap 2 in one hand, 3 in the other, and fluidly switch the rhythm between hands, while counting out loud.
Example:
RH 3, LH 2, count 3
RH 2, LH 3, count 3
RH 2, LH 3, count 2
RH 3, LH 2, count 2
Do the same exercise with 3 and 4.
You can also involve the feet while walking:
Feet (in 2) starting with Left, RH 4, LH 3, count 4
Feet (in 2) starting with Right, RH 4, LH 3, count 4
etc.