Then I started to shape the spoiler using some plastic strips and some wood.
You could consider this as a mold, the area between the edge of the roof and the plastic/wood construction will be filled with glass fiber reinforced polyester resin.
Of course I don’t want any polyester resin directly on my beetle, so I taped it all up. I used some play-doh from my kids to shape the corners.. 
Because this (painters) tape isn’t enough to protect the car from the polyester resin, I put a second layer of aluminum tape on it:
And because the aluminum tape is to slippery, I put another layer of painters tape on it:
And, after that, I put some plastic over the beetle, to elimnate the risk of accidentally dropping some polyester resin on the car!
I cut the plastic open on the part where the polyester resin goes, just like they do on an operation.
Then I bought some glass fiber reinforced polyester resin:
But it turned out I had to buy another can! Eventually I put 1,5 kilogram polyester resin in it:
When it hardened I sanded it down, and put some filler on it:
And I had to repeat that multiple times…sanding, filling, waiting for it to harden, sanding etc.. 
The result:
And all the tape and plastic remove, held on by my lovely assistant:
Now the edges and the underside had to be smoothed.
After making it all a bit smoother and nicer, I discovered the spoiler didn’t really fit nice to the rear window. Apparently the polyester warped a bit while hardening, probably because it’s a big mass of polyester. This is not going to work..
how can you fix it on the glass?
I will post a link to the installation manual here, when I get home from work.
Hi Haman,
You can find the instruction manual here:
http://www.gerrelt.nl/roofspoiler/handleiding/instruction_manual_Gerrelt_rear_window_spoiler.pdf
The spoiler’s lip sits under the window rubber, and two short beads of sealant are used on the inside.
Greetings,
Gerrelt.
That sounds like a vortex generator , which may induce more drag than it reduces. My understanding of the roof spoiler is to provide a sharper cutoff to the airflow to reduce some of the drag caused by air partially detaching along the steeper back slope. By detaching the air completely you still have the full drag of the entire frontal area.
No, I don’t think it’s like a vortex generator. I think vortex generators are more for directing airflow to a rear spoiler.
It works more like a kammback. See: https://en.wikipedia.org/wiki/Kammback
The angle of the rear of a beetle is not good for aerodynamics. A long sloping rear or a cut-off rear (like a kammback) is better.
But this spoiler acts like the rear is cut-off just behind the spoiler.
Hi on the top part you can install a thin piece of metal and put it under the rubber slip it in and this will work awesome
I too am wanting a spoiler. I am redoing a 72 sb. Were you successful in producing more spoilers?
Hi Michael,
Yes, I even have them made by a fiberglass compagny now.
Scroll down this page for more beetles with the spoiler:
http://www.gerrelt.nl/section-aerodynamics/aerodynamics-roofspoiler-production
Greetings,
Gerrelt.
Thanks, how do I get one?
Hi Michael,
Sorry for the late reply, but I was on a (short) holiday.
I will send you the details.
Greetings,
Gerrelt.
Hello Gerrelt,
sent you a request for your roof spoiler. Hopefully you will still produce them. Looking forward to you reply
Cheers
Marc
I’ve sent you an e-mail.
Hi Gerrelt , can you give the price about your roof spoiler please ? possible to get it in full carbon fabric ?
Thanks
Guillaume
I’ve sent you an e-mail.
Hi, I want to buy one for my vw 1303 , 1973. Can you contact me please? Thanks. (find me on fb)
I’ve send you a facebook message.
Hi! Interested in buying a spoiler for my 1303s 1973. Would appreciate information about price and shipping etc. greetings from sweden, thanks in advance.
Anton
You’ve got an e-mail.
Hi Gerrelt
I am all so Interested in buying a spoiler for my 1303s 1973. Would appreciate information about price and shipping etc. greetings from Denmark, thanks in advance.
Kent 🙂
I’ve sent you an e-mail.
I would like to order one of your rear spoilers. What is the price with shipping to the United States (Georgia)?
I have sent you an e-mail.
Hi Gerrelt,
I’m also interested in purchasing a rear spoiler. Need shipping to the Utah, United States.
I’ve sent you an e-mail.
Hi Gerrelt
I like to order one of these spoiler for my Dad 1970 VW Bug. the rear window has a defroster in it! How much and can I get one real quick? flying to California on the 18th of November
You’ve got an e-mail.
I’m interested in your spoiler. I have a 73 Super Beetle going for the German Look!
You’ve got mail.
Hi Gerrelt! I would also like to inquire about ordering one of your spoilers for my 70 standard. Thanks!
You’ve got mail.
Hello Gerrelt,
I have a 1973 VW Superbeetle. How do I order a spoiler?
Best,
Darin
You’ve got mail.