U bolt pipe clamps and flat bar
You own a Defender but not a welder, how rare is that....
When these were all the rage we would cut a jubillee clip in half, flatten it out, pop rivet one end to the top of the spot lamp and the other to the slam panel or bars. You can then screw the jubillee in and out to set the beam and they stay set.
use exhaust clamps and thin angle (dexion type) cheap. use the angle facing down and cut to profile of tube that will give a flat surface to mount spot lights to. Or remove the mounting bolt from the spots and use one of the exhaust clamp nuts instead that.s how i mounted roof lights to my first roll cage