Rebuilding engine room walls?

[Halifax Sailor]

Originally, the boat had a saildrive, like all saildrives nice enclosed area around the motor accessable by removing stairs. . Previous to me, it was repowered with a small Yanmar (ysb8) with prop shaft. Engine works great, but like all small diesels, very noisy. When the engine was replaced, a hack job on all the walls around the saildrive, just to remove and make room to install the diesel. So I am left with open space from behind the stairs to under entire cockpit. I would like to build walls, removable if possible, and line them with noise reducing lead/foam. I have been looking at either 3/8 plywood or abs board. Plywood I have worked with all my life, abs is new to me. I think for all I need, abs would be cheaper but I have to fasten it to the boat with "windows" so I can access the sides of the engine. Looking for advice and opinions whick, abs or plywood would be best for this application. I don't think I need marine ply as there is no water intrusion at all in this area, never has been. Ply I can tab into the hull and other sides, abs?? I don't know how it works.