The space mines in Collision Course were built in two scales. Two larger mines was made from plastic picnic plates (from the same set used to make the body of Voyager One). Strangely the smaller mines (10cm in diameter) are shown in close-ups. Even in space, the mine has three hooked feet, and six peripheral spikes, like naval contact mines.
Studio photo of the two scales. The detailing is similar, even down to panel patterns, but the larger one has wider sloped sides, with black panels that aren't present on the smaller version.
The larger model (pictures thanks to Alex)
Photos from the Alpha:2014 convention
Photos from the London:2024 convention
Photos from the Alpha:2014 convention
Contents copyright Martin Willey