Instead of using a dremel and getting rough edges, follow that guide up untill step 6 where there is a pilot hole drilled.
Drilling from underneath using that pilot hole as a centre for hole saw drill the bottom skin only, also make another pilot hole on the other cross hares and drill from underneath to begin with again only the bottom skin.
You will then have two big holes on the bottom skin and just two pilot holes on the top skin. Repeat that process on the top skin using the correct size hole saw to suit the aerocatch size.
Once that is done then use a dremel to join the two circles in the top and bottom skin and you will have two perfectly shaped tear drops, a snug fitting one on the outer skin and an oversized one underneath and I would recommend buying some of these as it makes it so much easier and stronger.
http://item.mobileweb.ebay.co.uk/viewitem?sbk=1&nav=SEARCH&itemId=370532307087
I also moved mine towards the centre as far as possible to stop them kinking the bonnet when there tightened down, most people fit them on the convex part of the bonnet and it ripples up the bonnet. Try getting them the right way round too, as if you have them the wrong way, there's potential for the wind to open them if not secured down.
I used the slam panel fixing hole on the N/S for the aerocatch pins and I made a plate up on the O/S that can be seen in the left of the below picture (it bolts to where tbe slam panel did) The slam panel ends on the car are different size, from side to side.