Use the largest sized panduit you can find that you can place the baseboard over, or use two for it. Only thing is, the kind with the velcro straps inside to hold the wiring is a little more expensive, but you can make your own by using a stapler to staple in velcro holding straps inside.
Also, carpenter nails will hold the baseboard fine on the panduit. I would first put up the panduit, place the baseboard on top of the finished floor, mark the height for the covers by using dabs of paint on the panduit, then when you pull the baseboard off, it will be marked exactly where the panduit cover is.
The paint trick also works when trying to locate junction boxes before cutting the holes into the drywall sheets, or plywood, etc.