You may be familiar with construction method.
The frames/bulkheads don't need to be solid. Often they have the centres cut out. This leaves room for fitting ballast, batteries, motors, RC gear etc. You can use a shallower keel than shown in the drawing supplied, but it may need to be a little thicker.
When the frames are cut out, leave extra material as tabs. These are then attached square to the building board using blocks. The hull is constructed upside-down. When the hull is completed, just saw off the tabs to release the hull from the building board.
If you look on the build logs pages on here you will see this method used several times.
Hope this helps.