Plastic\composite corrugated roof panels of course. Way cheaper than trays. Sloped to drain the runoff to the lower end into a gutter, or tote, res, etc.
PVC is the cheapest but also flimsiest, so you would want to set it on plywood\osb. Polycarbonate or other sturdier panels could be strong enough to span over whatever frame you build, without flexing much with the weight of the watered down plants.
You could keep the roof panels (two 8 ft panels overlapped a few inches could run the whole length of your room, down the center or along the walls) intact for longer sideways runs but would have more of a slant, the non drainage side would be a lot higher to get a decent slant.
Or rip the panels into 2-3 foot sections, overlap them from side to side, and run the gutter/s on the side/s of your 3+ ft isle way in the center of the room. Would need a longer gutter than way, whereas you could just drain into a 2-3 ft wide tote if you use the whole panel.
Many ways to configure the roof panels to work.