Foam is foam ... 2-ways.
1st, start hole wlth suitable hole saw so that hole guides the hole for your spade bit or new PVC cutter.
Option 1 ) Spade bit on extension handle can work, but the holes can tend to walk in foam. And if you don't have one, the extension handles are $20 or so. So you can do this below, just by using the same PVC tubing you will be using!
Option 2) I took a PVC pipe and hand cut some TEETH on the end to make it look like a hole saw. 10-minutes work tops!
-Wrapped other end with duct tape until it just fit into another hole saw, then taped them together with duct tape - wonderful stuff, haha! Then hook up to cordless drill.
-Use new tool - insert into the hole already started - drill - pull back and clear chips, repeat - drill again and clear chips.
-Also used a long handled screwdriver to 'snap out' and remove cut sections.
-But if you're really going in far ... best bet is to keep it one long hole, but you will need to remove the tool to clear the chips (chips make the tool cut off-line ...) and then when at the depth you want ... wiggle, wiggle, wiggle it ... so hopefully she breaks off completely at the far end.
Tip - water can be used a cutting lubricant. I drilled 18" deep holes in a foam V-berth bunk one using the homemade PVC hole saw, but I used to be a machinist, so maybe I had some learned benefits on cutting tools ...