Depends on what you want to spend. You can get a good 3/4 inch aramid core and glass it on both sides. Closed cell core is what you would want. It may be a little tricky if you've never done it. Most of the time this kind of construction is vacuume bagged, but you can hand lay if you do it right. Most floor panels in aircraft are constructed with aramid cores and thermoplastics/thermosets. Or, for cheaper and easier, just glass over your plywood.