Ok, so planets are flat. Sure why not. What if, instead of a flat bottom, the planet domes downward?
With lava pooling up around the bottom and maybe even a nice thick crust block that is hard to shoot through and can't be mined up? This'd outline only the bottom of the planet so people can't just go mine up from underneath it, they'll have to land on the planet or mine from above it.
Mostly, this suggestion is just to dome planets at the bottom.
Oh, and when planets are formed, there should be no visible ores or lava on the edges, just plain old rock at least 2 to 4 blocks deep all around.
With lava pooling up around the bottom and maybe even a nice thick crust block that is hard to shoot through and can't be mined up? This'd outline only the bottom of the planet so people can't just go mine up from underneath it, they'll have to land on the planet or mine from above it.
Mostly, this suggestion is just to dome planets at the bottom.

Oh, and when planets are formed, there should be no visible ores or lava on the edges, just plain old rock at least 2 to 4 blocks deep all around.