There is "lightweight" MDF available. I've used it. The regular stuff weighs in excess of 90 lb for a 49x97" sheet (roughly 3 lb /sq.ft.) The light stuff is about 40% lighter.
A strong and lightweight solution is a "torsion box"
Clicky for procedure for a heavy-duty version If you don't feel up to making one, you can get a hollow core door with the eggcrate interior and laminate that up. Though you'd need to beef up your attachment points on the bottom.
If you decide to make a torsion box, you can get "door skin" plywood and build your box with that. Then laminate with HPDL. The only caveat is that you have a thin surface layer so don't be dropping heavy sharp objects on it.