I have heard for years that cutting the 4+ Inch foam toppers, mattresses, new cushions is like cutting butter with an electric knife. Having no electric knife and being far removed from my Dad carving the Turkey with one we got with Green Stamps, I have slaughtered the edges on many over the years.
Needing to cut and custom fit our new 8" Spa Sensations mattress for the permanent bed, I finally broke down and ordered Black & Decker EK700 9-Inch Electric Carving Knife, White Heck it was only $11 and Prime so no shipping. Last week I brought the mattress inside and marked the dimensions on it including the curves on the back wall. I fired up the new knife and within no time I cut through that foam with a near perfect edge like it was a hot fresh loaf of bread. I will never cut foam again without my trusted Black & Decker at my side.
FYI, for ordering new foam for cushions I have always found this supplier to be terrific on quality, price, and shipping. Rochford Supply