Cheap DIY Subwoofer Pad Isolation Riser

I apologize if this has been asked before, I searched and couldn't find a specific answer.

I am looking for something to put under my sub in my apartment to reduce the bass transmitted through the floor so that I hopefully don't piss off my neighbors. It's a front firing Logitech Z5500 sub.

Searching around I found that the Auralex Subdude/Gramma is probably what I'm looking for. But I'm cheap and my system isn't worth that much so spending $50 on a pad for a set of speakers that I got for $180 isn't really worth it. Is there a cheap DIY solution that I could make myself instead? I was hoping I could buy some foam from Home Depot that is similar to the Subdude but cheaper.

Any ideas would be much appreciated. Thanks.

Thanks for the tip. I know that I'm not going to be able to completely muffle the bass that my neighbors can hear, but anything to reduce it at all would be nice.

What exactly is underpad and where would I find it?