Hello, :) I have scratch built a few subs using a male plug then female molds.
You are about to contemplate doing a great deal of hard work in order to get a working mould. First I decide what access you require to the hull and where to put that access, as this can determine how you mould the fibreglass, ie longitudinal separation of the hull etc.
After I have decided how I am going to get into the thing when completed I build the two plugs ie tip half and bottom half, on a building board with bulkheads cut to the correct dimensions glued vertically on the board then I cut planks of wood and attach these to the bulkheads ( as you would when building a plank on frame hull ).
When the shape is correct you sand them very smooth and I then add details I want on the hull ie hatches rivets etc onto the two halves then when its all done you need to varnish/ paint /seal the wood again making sure it is all smooth ( any defect here will transpose itself onto the mould and finished product ).
Then you need to wax the plugs and about 2 " around them on the building board. polish this then do it again the again etc until its nicely polished.
Now you can think about the fibreglass bit, you need some wax, release agent, some fibreglass tissue, fibreglass matt, Gel coat and resin and hardener, The cheepest way of obtaining this lot I found was to source a commercial fibreglass component manufacturer and go and speak to them explaining what you are doing and ask if they would sell you the stuff, most are small buisnesses and mine was very interested in what I was doing and provided the stuff I needed at a fraction of the cost ( they use this stuff by the 45 gallon oil drum quantity ).
Now the problem is this, you have to get the moulds off the plugs so you have to decide where to split the mould, if its a simple hull ie top and bottom are smooth and round ( like a modern sub ) then you can fit a lip at the centre line of the plug running from front to back about 2 " high, I hold mine in place with a hot glue gun on the second side i am doing, this is where the first mould will be made. You wax this lip as before then use the release agent wipe it on with a cloth ( just one way or you will wipe it off again ) when this is dry you can start to lay up the mould, you paint on the GEll cote all over the first half of the plug on the 2 " boarder and lip give it a good coat let it go hard then use fibreglass tissue cut into strips that you can easily manipulate, place these onto the gell the with a brush you STIPLE the resin into the tissue DO NOT TRY AND PAINT IT ON you will end up with it stuck all on the brush, do this all over where the gell coat is and let it go off I do this about 2-3 times to get some strength on then use fibreglass matt to beef it up . when all dry give it a day.you can then remove the 2 " lip that runs down the center of the plug, it should come away easily if waxed and release agented ok. now you do the same on the other side of the plug AFTER you have waxed and put release agent on the 2 " lip of the first mould this is where they moulds will split, when dry drill several holes through this lip this is where you bolt it together after it has been removed from the plug. Now you can remove it from the plug I use a wide walpaper scrapper to do this you place it under the fibreglass on the building board and twist along the joins and between the two 2 " lips and you should here a loud crack as it comes apart. you do this for top and bottom hulls when removed you will have a four part mould ( 2 top 2 bottom ) which bolt together all you now do is bolt them together wax the insides of each mould, release agent them , then gell cote the entire insides and lay up as before leave to dry and unbolt the 2 halves and prise apart then you should have a top and bottom hulls. The problem is when you have several tight concaved areas to do as you will have to mould these parts individually as if not you wont get them out of the mould and the Seaview is a bit complicated, and VERY big, why don't you scale it down ??.
As I said at the beginning it is a lot of hard graft to do this hull...but this system works and has worked for me...now you know why people buy Hulls already done !.
Standard boat hulls are easy to do but working subs ? Sorry if i have blown your mind you did ask...