Felt pad for shower install?

A 60x30 shower is a pretty decent sized shower. Most shower pans have the "floor" layer that recedes to the drain and the slope leaves a vacant/hollow layer underneath the pan.

I have only seen a handful of pans in my life that actually sit completely flat on a floor - it is usually just the drain part and the outside.

Felt is used when a pan does sit flat or if there is a bed that the pan sits on that is already "curved" for the pan. Simply for a basement floor I would never put a felt pad down.

There is a good chance if you did just put a pad down then the pan would feel hollow standing on it. And the other issue is that depending on how the pan is made it could "flatten" (yours would not flatten but the mortar would allow you to correct the drain height) and not drain right. Not a huge issue if the pan top is plastic but if it contains cut granite/tile/whatever it may not dry out properly.

My advice would be to pack in the pan to achieve a proper drain slope. The mortar should be solid to pan and I would estimate fill 90% of the hollow area - you don't want it just spilling out. Doing this right can make a cheap shower pan seem expensive and solid and its just the right way.

On a side note the mortar and bucket is probably cheaper than a felt pad. You want to put the mortar in right when you are hooking up your drain.