@zacharyj840 full disclosure I don’t have a frag tank — although sometimes I wish I did. But here are some thoughts for what they are worth.
So the focus is on flow and lighting. In terms of flow most low boys I’ve seen have the returns on one end and the overflow on the opposite. So you get high flow to low flow across the tank. Then I’d map out PAR. My guess is that your edges will be lower PAR and the centerline higher.
In terms of racks, assuming it’s not too deep, I’d just go with egg crate panels in several sections so you can move them if you need to clean the bottom of the tank.