Ditto on the cheap electric carving knife from a thrift/Goodwill/St. Vincint de Paul/etc....
They cut foam almost too good.
There are a lot of various densities (firmness) of foam to choose from. I/we like a memory foam "topper" to sleep on (Costco) over the main mattress/foam.
I you have a local overstuffed upholstery re-upholsterer you can consult, they should be an expert at judging the type, density, and quality of foam you want for your mattress when you tell them what kind of mattress firmness you sleep well on. They might even give you a good deal to buy your foam through them.
And they could wrap and sew a covering on the foam with however a high end fabric you choose.....
My upholstery guy loaned me his electric knife and gave me a can of spray foam adhesive plus various foam densities to glue the different layers of firmness together to get exactly the "feel" I was trying to achieve on an upholstery job. Then I brought back my foam creation, and he wrapped it in the fabric I chose.
Admittedly this was small town customer service at its best.....
