Commenting here on Ryan's original opening post for this thread:
Ryan mentioned using Tygon tubing, but I thought I try something a bit more common, so I picked up a chunk of 7/8'' o.d. (about 22mm, I don't know what sizes are common in metric) vinyl tubing from my local friendly hardware store, and it works quite nicely to adapt the Instaflator to work with the Exped's air inlet.
My chunk of tubing was kinda flattened into an oval cross-section, as it had been taken from the innermost wrap of a nearly empty reel, so I pitched a 2'' chunk into boiling water for a few minutes to let it relax back into shape.
The Instaflator's outlet fits snugly inside the tubing, and the tubing's o.d. is just right to snug tightly into the mattress's inlet.
Easy as cake. Thanks, Ryan!