You could get a couple of large plastic trays just to keep the plants in

Leave the wrapper on the rockwool cubes and use something to cover the open surface at the top to stop algae setting in.

Space your plants out in the tray and feed them by having a bucket of feed near by and dunking the bottom third so it soaks the water upto the top, you want the rockwool cubes to be damp not soaked.

250w could be enough to veg the number of plants your looking to do what kind of light did you have in mind