I dug in my parts box and found some terminal blocks that look to be of similar size (5 to an inch).
Looking at the Mouser catalog, most of the 5 terminal blocks at 5mm pitch take a max of #14.
A piece of #14 solid fits just fine and makes good contact. I can't pull it out by hand - I tried!
That should be good enough to switch a single flood.
For switching multiple floods, I'd have the ELK relay engage a contactor.