Commercial hatchling racks I've seen made from what I believe is HDEPVC (high density extruded polyvinyl chloride) still seem to be designed to have the short dimension accessible and only 3 or so across.

I personally don't know how melamine coated wood will hold up in terms of failure level sagging over time. I have seen others stress the importance of keeping moisture away from the exposed wood portions of these boards because they can absorb it and expand in a way that results in permanent damage.

When woodworking, there is a system of measurement designed to assess stiffness which rates different types of wood on their ability to withstand a load perpendicularly applied to the grain (how you'd most likely use lumber to build shelves, etc.) Choosing a hardwood is one way quality furniture stands the test of time.

I believe the person from Olympus Reptiles on Youtube has a home constructed side access rack system. Maybe you could message him?

Good luck, hope this helps!