I bet with a little creative fabricobbling you can come up with something to clamp that heater in place. Maybe a tab mounted to something else to give a longer screw something to hold on to. It doesn't need to be anything amazingly stout, just needs to keep the thing from rotating or falling out. Depending on exactly how much space is available behind where that thing mounts, maybe carefully drill through the inside of the heater mount area, use a couple of small screws to attach a tab to the back side of the mount and have that tab stick out the side where you can secure the heater. Just have to make sure the screws and metal bit won't interfere with anything in the high idle mechanism.
If you won't drive it in the cold and don't care about this, just use a piece of wire to hold the high idle cam out of the way. If you want a bit of a hacky in-between, you can wire up a switch to manually control the throttle kicker and get an idle speed bump for cold start use.