stacking components without leaving marks

It's best not to stack components for more than one reason.

Paper works. I assume that you are going to use stock rubber feet. I stock components too, but I use different feet under each component; the difference in sound when using good feet is quite big.

Every shelf in my main audio rack has a pair or even three componets stacked. There is NO reason not to stack components IF they:
Do not radiate: too much heat or too much EMR.(electromagnetic radiation)
There are no other reasons, other than looks.
Usually tube amplifiers and tube integrated are the too much heat. Class "A" running solid state amps too.
Everything else is fine. I tend to keep the Phono away from the Digital, but that is just me.
Added: No stacking was invented by the folks who make or sell component racks for a living

Felt pads for furniture works. But I do like Lizs comment.

BTW... Don't recording studios stack most of the equipment.

Elizabeth, Not true. When you stack components you are mass loading the component on the bottom which will affect the sound.