Reconstruct missing part of Hard Top

I have a project where I need to reconstruct parts of my HT. See picture.
The back of the HT is cut by a previous owner, and since I am new to fiberglass work I'm looking for advice how to do this the best way.
The curve for the missing part is the same as the existing HT. At the end of the reconstructed HT there will be mounted a curved teak list with the right curve. This teak list I have got brand new.
My idea is to use a plastic sheet or something and clamp it to the HT to create the right curve. And then fiberglass on top of this and remove the plastic afterwards. But I have not found anything suitable to create this "mold"
Does any of you have some tips for me? Can I do it like this?
Staale

 

You need to make a mould.

There are people here far wiser than me, but here is what I would try.

First, make a template of the existing top using cardboard.

Then, cut two pieces 3/4" thick plywood or pressboard or mdf into that shape and check it for fit. The templates ought to be flat on the noncurve side.

Then apply the templates to a rigid board and put spanners or bridges in between to keep the templates from twisting moving.

Then apply a piece of formica to the jig using contact glue.

Then you can build the piece.

If you want to use gelcoat; I can't help you, but others here can.

Wait for a few more ideas.

You could also lay thick plastic over the boat and use the existing hardtop as a plug, but the piece would be a little big that way. It all depends on how nice you want the fit. You will end up having some ridge to connect the repair as it can't be a simple butt, so a slightly larger piece could go over the top and side edges. If you want it to look best from the outside; you could do the attachments on the inside only using a plug as I first described or you could fair the overlapping large boat mouldsd piece and not see it from 20' away.

It would also help anyone advising if you did a closeup of the area. It is a bit hard to see in detail.

 

Close. Probably the best way to do is to create a jig of 3 or 4 rib-like forms that go from the cockpit coaming up to the profile you want for the roof. Attach them together with stringers. With a little heat and patience, thin (2-3mm) acrylic (or any other plastic) sheet should be able to take that radius and will be rigid and smooth enough that not a lot of fairing work will be necessary. You'll probably need to add the complication of a lip to the bottom of it to give a bit of overlap to the existing HT, because otherwise its likely to crack and fail at the joint. Then dismount the jig and glass away!

Have you considered that maybe the previous owner cut it back for good reason? A soft top bimini-like canopy might a lot easier and flexible?

 

Personally, it looks terrible to me the way it is, but a higher bimini might help

 

Thanks for your good ideas. I did not think of making a mould/jig. The previous owner had mounted a windscreen on top of the HT, and cut it to be able to drive the boat standing. I want to fix it as close to original as possible.
Attached is a closeup of the problem area. And also a picture of how I want it to look like.

 

I would build a jig from underneath like James advised. You can clamp it to the existing hardtop and make sure to allow for the mould surface thickness. Screw it together and make it so you can move it to a table. Use a lot of screws so it doesn't lose its shape. Multiple screws in all joins and extra reinforcing. You could do the work on the boat with tons of plastic and shipping tape. I'd prefer to move it to a table.

You'll need a tape seam at the inside that will show most likely for strength.

Where the two parts meet on the top; you can sand the seam down about two inches on each side and about 0.050" deep or 1.5 mm and install a fiberglass tape that will be in the relief.

It'll be fun.

How thick is the existing top at the area not cut?

You'll need to match the thickness pretty close.

It looks like there is a step?

If you report the final thicness; someone can help you with a glass schedule.