Bought Luxury Firm Foam 6 inch thick to restore mid century sofa seats.
Cut them by electrical kitchen knife with no issues. The size of cut seat pads shall be a bit less (about 1/2" less) than the original cushion pad made of yellowish soft porous material of 20th century.
Overall result is wonderful, the firmness is what we expected to have, no much deformation under the weight comparing with the original pad. The new pads just hold weights providing a comfortable support.