The old type7 hull was usually fixed this a lipped locating plate t the front, when the upper hull was placed on the lower hull it is slid forward so the plate in the upper half hooked under the plate in the lower half. The aft end was fixed with a nut screwed down onto a threaded post which was fixed to the lower hull and projected through the deck.
Here is a modification and more suitable method which is employed on the Type2D:
Mount a plate in the bow of the upper and lower hull halves. Make a brass pin which will be fixed into the lower hull plate and enable it to locate in a hole in the upper plate. Perform a similar act at the aft end of the boat. This will allow the hull halves to locate fully.
A method for holding the top half to the lower hull:
If your dive unit is cylindrical, make a strap which goes around the central tank. make brass block wiith a vertical brass post long enough to arrive somewhere just under the tower and drill thread the top of the post to accept an M3 bolt. Solder the brass block to the top of the dive unit strap with your post vertical. If you drill through the tower deck, the screw will engage with the hole in the vertical post once it has been alinged. The strap on the dive unit can now be tightened and when the top is placed on the boat, it should be permanently aligned. When the screw is located and tightened sufficiently, it will pull the two halves together. This method works and will prevent a gap appearing between the upper and lower hull halves. The screw in the tower can be disguised underneath a lifting hatchway so there is no evidence of how the top is fixed down.
I hope this helps.