Though it's not the best looking attempt, it's not exactly the wrong way to do it.
You can pretty much do it what ever way you want and if it's for a personal one off production which in the end works and looks good then good on you.
But if it's going to be a product which is to be sold or reproduced then a lot of refining will have to be done such as making it fit perfectly and interchangeable with the original fixings of the car.
In other words you'll probably have to make a mould off it and make 1 piece pieces like I said before.
There's nothing wrong with using FRP and fillers, it's not going to fall apart at 60mph. Looking at most custom production cars, they're just a rolling cage with a FRP 1 piece external shell kinda like a remote control car.
I wouldn't say riveting brackets and panel welding is a good way but it is a way of doing it.
A Friend of mine who does welding would look for an arch off another car and transplant it.
Take a look at this. It was a golf with BMW flared arches transplanted. Just cut out and spot weld on.
http://www.ramscustoms.com/vw_golf_mk3.html#With weld on, the down side is that it's still heavy but looks original. Also in time it does rust out from the underside quicker than normal even if it's sealed.
Then you have the option of newer techniques such as using expanding foam which we've seen before. Most concept show cars are done that way.
I've even made things out of card and masking tape then fibre glassed and refined before making moulds and then produced.
I'll say keep up the work and lets see the results first after all it's still in the making.
And please for the love of GOD go back and learn 2nd grade English. Thank you.
