OK if you are intent on doing the work yourself, here are some suggestions about how to go about it. I have not cut a vent in such a hood myself but I have reworked various vintage hoods when they have been damaged, bent out of shape, scratched or dented. I also do a lot of small metal work for hobby purposes so I know my way around hand tools of this sort.
Getting the correct size and shape of the cut out initially involves a little trial and error. You should draw up a template for the cut out and mark it on stiff paper card. Experiment till you get it shaped to your satisfaction. Then carefully cut the shape out of the card and glue the resulting template to the back face of your hood using double sided mounting tape. This will indicate the dimensions of your cut out. It might also be useful for the final fitting stage to use a paint based fine (1mm) marker to mark a white line around the edge of the template. This will help with the final fit. If you do not do this and just "eyeball" the cuts the result will be hit and miss so this step is important if you do not want to risk a bodged modification.
I think I would then be inclined to then use a power drill (a bench drill press is best) possibly putting at least one large hole in the middle of the cut out to get rid of the bulk of the excess material or just going directly to putting one smaller hole in each corner of the cut out - staying within the boundary of the paper card template - you need plenty of room for final fine fitting.
The best way to cut around something like this is using a jewellers hand saw and metal fretwork blades and cutting from one hole to the next as made in the above stage but a Dremel power tool will do in a pinch. Though in my experience as much as I love Dremels for certain work they can cause a hell of a lot of havoc in a very short time if not ultra, ultra careful and dextrous. In any event again I suggest making only a rough cut out the shape being careful to stay well within then boundary of the template at this stage. If you work from a large hole in the middle you can go straight to jeweller's needle files to file away excess material to the edges of the template instead of a Dremel. This is safer though slower obviously.
A set of Jeweller's needle files which can be found cheaply in good hardware stores or online. Just use one or more to carefully enlarge the hole or preliminary cut to the full size of the template using the white line marked on the hood as the final guide. Use 400 then 800 grit abrasive paper for final clean up of the cut to get rid of filing marks - backing the paper with one of the needle files for a clean and flat finish.
Assuming the hood is aluminium (I have not checked its composition for your lens though the hood for my pre asph 35mm f2 was plastic - which means neither painting or chemical coloring is needed and you can ignore the next two steps).
If the hood is metal, touch up the cut edges using black paint. I suggest going to a modellers store where they sell model planes, model military gear etc and buy black paint in the correct shade and level of gloss - they typically have many variants which makes it easy to match. And they come in tiny cans plus the store will have the requisite ultra fine brushes. Don't use spray stuff in this case.
An alternative to painting is to go to a gun store and buy something like Birchwood Casey Aluminium Black from that source. This is not as robust as anodized aluminium but seems to stand up to bumps and abrasions much better than black paint and is an easy way of touching up scratches etc on metal hoods and lens barrels.
If you want some idea on how to hold the hood and do the above work check out the video below of a jeweller's work bench. See the little wooden thing hanging off the side of the bench with a V notch cut in it. Well that's what you need to make (you can do it quickly out of MDF board which can be temporarily clamped on any convenient overhand such as a kitchen table if your wife is compliant
🙂 ) To use it you rest the work piece / hood on the top of this so it supports both sides of the hood while the needle files (or jewellers saw) work up and down in the V notch. This provides a firm work surface which allows fine detailed work.
At least that's what I would do.
https://www.youtube.com/watch?v=2x5aViE1dVQ
