To match sensitivity levels, you can pad the horn with a 4 ohm resistor in series and an 8 ohm resistor in shunt. This brings the sensitivity down but dynamic range is still increased by horn loading. And drive requirements are less, so distortion is reduced too.

For a spacer, you can sure use the spare MDF you're talking about, but that requires you to cut a ring. Another way to do it is to use an extra gasket as a spacer. They don't cost much and most recone shops have them.