So I’ve seen numerous people recommend 1in foam insulation. My Lowe’s doesn’t seem to sell that. They have 1/2 and 3/4, but the 3/4 seems to always be sold out. They do however have this:
Would this be thick enough? Or should I double it up? Or does it work at all?
That is commonly about 1/2” thick — I think 1/2” should be plenty with feet removed.
If you remove the feet on an XXL (or similar), your frame will sit directly on the table below. So, any thickness of “foam” will hold the machine off the table.
There is a caveat to that. There are mounting screws that extend below the bottom of the frame members, so you have to have enough “foam” to so those don’t reach through the “foam” and touch the table below.
I’ve had no trouble with my XXL frame sitting squarely on the table without “foam.” I just created pockets where the screws don’t get in the way. YMMV.
Depends what you want to achieve with the foam.
Are you trying to quieten the machine or deal with a sagging or deflecting spoilboard?
Sagging/deflecting, mainly.
OK,
Then you probably don’t want a soft foam for that then.
If you have a suitably flat strong surface to put the Shapeoko and Foam on then I’d use one of the rigid insulation foams. These are relatively inelastic and quite strong if you spread the load over a large area.
With a sheet of rigid foam you can do as Tex suggests and cut out holes for the screws on the bottom. You might also cut slots for the bent down edges of the main front to back rails to get the whole flat base of those rails to sit on the foam and spread the load out evenly.
You can either remove the feet or just wind them back up to clear the table once the foam is in place.
Does that make sense?
From what I understand of the foam deal is for the machine to settle and basically self level. Everything I’ve seen says to take the feet off, slide foam under and just let it sit. I understand that EVA is softer, but it is still pretty firm. If I’m wrong, feel free to correct me. It has been a very difficult topic to research.
I used this and it works great for me.
https://www.harborfreight.com/4-piece-anti-fatigue-foam-mat-set-94635.html
Looks to be the exact same as the Lowe’s stuff for half the price. Did you only use one layer? Also you’re using it with the feet removed?
I always cringe when someone says Harbor Freight is half the price of somewhere else. The price difference is true but is the quality of the product true as well. I find that most stuff from Harbor Freight is the bare minimum specification to call the item what it is called. There are a few things at Harbor Freight that are decent but most of it is so shoddy it makes you cringe. So not to compare apples to oranges but would you want to be hanging over a lava pit with a rope from Harbor Freight or a good old heavy duty rope over rated from its specifications. Dont look down.
I mean, foam is foam though?
Just to be clear, you aren’t technically trying to get everything “level”; just supported. I wouldn’t want you to be trying to fiddle with level like you would with a door frame or a game table. 
PS. I should have mentioned before that my machine rests on the top of a torsion-box table, so that’s the main reason I’ve never put “foam” under it. It just stays flat.
I only used one layer. I removed the leveling feet. It is very dense and supports the weight of the machine nicely. I use it on a torsion box cabinet I built. The main reason I used it was to dampen vibration. It works very nicely for me. People say what they want about Harbor Freight and I agree some of their stuff is crap but there are many things you can buy there that are decent quality for minimal cost. Can you say zip ties 
Just dont buy power tools expecting them to last a lifetime. If you need a power tool for a one time use, save the money and buy from them. If you want something to last more than one season, spend the extra money and buy a quality tool. As always, BUYER BEWARE!!! This holds true regardless of what you buy and from where. I have my favorite brands but for this application and using these pads for anti-fatigue floor mats, it is a decent alternative that worked well for me.
Almost 3 years with th XL & XXL
Never used any foam, always been on the 4 feet, I’ve not had any sag…
Or…
fully assemble the machine and board. Square the machine frame. Remove the feet and drill holes to place deck screws through and screw the frame down to your table = as solid as your table.
Remove the supplied spoilboard and supports, place them with the now unused feet.
Now, build up the surface of your work table to meet z-travel and work area.
Many benefits to this and no downside (so far).
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