A panel saw will be easier, to keep on a straight line. Seal the sawn edge after.
Assuming there is a rim it needs to fit under, get it to go as far up the rim as possible, even if it needs to be notched for hand bracket bolts under the rim. Wedge it as high as it will go, then measure down from the bottom edge, to determine where the plinth need to be fixed, on the bottom/rear.