Same experience here, the bottom of the MJ3 runs uncomfortably hot if not given some added space.Far be it for me to disagree with Jason on something he says about his own products, but even the Mjolnir 3 on top of the Gungnir gets borderline too hot for my taste. At least without spacers, the Gungnir's metal heats up quite a bit with a Mjolnir 3 on top. That's not the Gungnir being unable to dissipate its own heat, but getting heated up by the Mjolnir sitting on it. This might not necessarily harm the Gungnir, but it's not great, either.
With spacers, however, it's fine. (I've been using cheap Amazon-sourced large-ish cork stoppers. Others like to use HVAC anti-vibration pads.)
But yes, anything on top of the Mjolnir is definitely a bad idea.
Using spacers like the cork or rubber/cork ones underneath the MJ3 when stacked on top of the Gumby is a good suggestion, and I use the same approach.
Bad idea or not, I have a Folkvangr stacked on top of the MJ3 using the same rubber/cork spacers.
Thus far no issues, and the surfaces all remain touchable except for the heat sinks.
It should also be noted that I don't run both the MJ3 and FV at the same time.