I used slotted adjustable rails and L shaped adjustable book shelf parts from Home Depot. Then cut my own boards. I added a front lip to each shelf.
Two rails on the left and two on the right. This only required a few screws to hold the rails in. And if the shelves are cut tight enough the pressure of them against the rails helps hold the rails in. They are also adjustable. If you get the long rails and is tall them facing the same way you can let them rest on the floor of the closet, for more support. This will
require cutting off the tops.
Be aware that the shelves are not entirely rectangular because there is a corner wire way on the left side rear that covers cables. This is an older picture but if you look close you can see the rails on the left side.
2014 FR330 DS