Using an optical switch instead of mechanical type, this advantage is no springs reaction force formed when retracting the probe. You can write about a g-code instruct effector downward to rise the probe, and then attracted on the top side by a magnet.
1). Mount - printed layer 0.2, infill 30% be enough.
2). optical switch - I think easy to find as the figure shown type.
3). M4 wrench - Across flat is 2.5mm, 60mm long, must metal type that be to attract by the magnet.
4). 10x5x2 Magnet - Normal not need strong or N35
6). M3 x 18mm x 2pcs
7). M3 nuts x 2
BOM also as figure shown.