I used these for my normal SCE dudes. Used two to four for the big stuff. Just dabbed a bit of plastic putty, dropped magnet, more putty, done. They all stick well to my carrying case. Obvs the drill bit isn't relevant to bases, but useful for using them on other stuff.

MagNetSol Magnets + Drill Bits Combo Pack (100pcs 1/8'' x 1/16'' Magnets + 2pcs 1/8" Drill Bits) https://www.amazon.com/dp/B0753M1ZYQ/ref=cm_sw_r_cp_apa_i_lvk-CbW2F6Q9K