18x18? 24x24? obviously diameter in this case...
check out thos spider farmer lights, might be good for that footprint, and an efficient option. it'll save you money every month you use it compared to many other options at roughly the same price.
compare efficacy.. if they don't show it, assume it is horrid. umol/s per joule or whatever... lol.. it'll stand out as such.
get something with samsung lm301 diodes and you'll be rocking. even if slightly more now, it'll pay for the difference in 1 year or less, in this context.. less i bet, compared to the cheapies you can find, plust grow better bud clsoer to 1.5g per watt.