I think there is something funny about the size, the ATV ones don't fit. Like you have to get the Big Red ones. Hopefully some one who actually knows will chime in.
I bought a whole frame plug kit on ebay that included the center caps. I want to say it was like $24. There are open ends on the round and square frame pieces in front and rear that will let crap in. Found it. GENUINE HONDA PIONEER 500 FRAME AND OEM WHEEL CAP KIT
The center caps off a Honda Rincon will work, I know this because I took them off my Rincom and put them on my P5 and I think the Formans are the same.
My p5..,didn't have any....on it
..but of all my Atv's..of many
brands..stock wheels & tires...
caps never stay on..the best was
a...yamaha big bear..,300.they were
metal caps..