M_Boats: Below the Cabin Floor

Martin Johnson, owner of "Sunflower" in Duluth, MN wrote an article in the MSOG website about the cavity below the cabin sole, and the keel of his '76 M-17. Seems he had CB trunk leaks that threatened further freeze-related damage. So he put inspection holes in the cabin floor next to the CB trunk, and filled the cavity with antifreeze in winter. Here's my question - Does anyone know how big the cavity is below the floor? I plan on drilling a hole and filling the cavity with two-part closed cell foam to exclude any water intrusion - even though I have no evidence of a problem at this time. It would be nice to know how much foam to mix... Thanks, Mark House 1977 M-17 # 232 "Seranita"