That is just the way I am doing it.
But I personly prefere to use a wing made out of fiber glas and just making the edge much thicker so that it is abel to flex without breaking. This saves weigth.

Just cut 'em off. Don't worry about the edge. Not a problem with the MOT man over ten years...

Done that

I just cut the rear wings on the 4x4 and then ran a linisher along the edge to take the sharpness off. Then is was couple of coats of paint (by hand) and the sharp edge is gone!!

The following was justed posted by me on the Aussie Frogs site, but I thought I would post it here as well before I forget all the details. It covers two modifications I did to Daffy Duck - firstly adding Toyota Landcruiser FJ42 foot vents and secondly adding a wheel well to the bonnet. Note that the foot vents consist of a triangular flap and the surrounding metal that it sits into (flap well), plus a hinge pin and spring to hold it open/closed).

Foot Vents

I glued the Toyota flap wells to the exisitng 2CV panel after cutting a triangular hole (using the flap as a template). The glue(s) that I used was Loctite 324 Adhesive and Loctite 7075 Activator. The adhesive was applied onto flap wells and the applicator was applied inside the 2CV panel. I had already predrilled holes in both (2 per side of the triangle) and when the two metals joined, I inserted rivets to position and clamp them. After they dried, the rivets were ground off, the edge between the 2CV panel and the Toyota flap well was filled with bog and sanded smooth.

My preference for gluing was twofold. Firstly because the Toyota metal is much thicker the 2CV metal, making welding difficult. Secondly, metal against metal is an invitation for rust. Unfortunatley, the two Loctite products are very expensive, but very good. I will get my moneys worth over time when I glue metal in the future (works well on joining aluminium to steel).

Wheel Well

I have some photos of Daffy Duck where I added a wheel well on the bonnet. The measurements that I found effective was:
1. Get a NEW tyre (without worn tread) of the size and brand that you intend to use. Add 20mm to the diameter and cut a circle from sheet metal.
2. Get 75mm width of sheetmetal (same guage as the circle) and have a 7mm lip bent at right angles. The length needs to be about 100mm longer than the circumference of the circle (diameter X 22/7). This will be the side of the well.
I forget the guage of the sheetmetal I used, probably 0.9mm. I bought it from a sheet metal factory, so they cut the circle and they put the lip onto the side metal. They also suggested the size of the lip so that it wouldn't ripple when bent around the circle. Remember these guys are experts at working sheet metal.
3. Use the sheetmetal circle as the template for marking the hole to be cut on the bonnet. I started 100mm from the hinge.
4. Tack weld the side to the base, with the lip under the circle for strength and a neat job. A tack ever 75mm gets the side bent around the base. Cut the end of the metal to allow a 40mm overlap. Then weld between the tacks.
5. Mark a line 55mm up the side from the base. Set the well into the hole in the bonnet. The 55mm line should line up at 12 o'clock (front of car), 6 o'clock (windscreen end) and 3 0'clock (right hand side) and 9 o'clock (left hand side). Tack at those positions. Then turn the bonnet upside down weld all around (most welding is on the engine side). There will be up to 20mm of metal above the bonnet that needs to be cut off and ground smooth.
6. Because of the welding, bits of the bonnet will have distorted, so a lot of filler will need to be used to get a good result. Remember to use seam filler both inside the well and under it (where the lip joins and where it is welded to the bonnet).
7. Buy some stick on under bonnet sound proofing and cover the wheel well and bonnet.

I have found those dimensions very good, in that the wheel well doesn't touch the aircleaner, or anything else. Being close to the hinge, there is no buckling of the bonnet with the weight of the tyre. The tyre is held in place with a round headed cup bolt through the centre of the wheel (remove the hubcap nut first), then it goes through a hole at the bottom of the well. A washer is welded on that hole for strength. Finally a wing nut underneath (so I don't need a spanner to undo it). I don't have any drain holes. If it has rained overnight, I open the bonnet and let it drain. When driving, the airstream sucks out any water.

Both the wheel well and the foot flaps look great on Raid 2CVs and are very practical. Good ventilation and a spare that you can get to quickly without unloading all the camping gear to get to the spare at the bottom of the boot. It also balances the car by adding some weight to the front, rather than all weight loading up the rear.

Daffy Duck

Good on you mate for posting this. I've seen it and its done very well.
Cheers bastard