The problem with putting anything, including wire mesh to keep the birds out, in the mouth of the velocity stack is it will screw up the airflow, causing turbulence, thus loosing the benifits gained by using the stack. The best, if room is available, is to put a high flow air filter around the velocity stack, this will allow the air in, and allow the velocity stack to work it's magic.
I realize that the stack in a filter is a problem for bikes, just don't have much room. I'm no wizard, most of this comes from a book "Tuning BL's (British Leland) A-series engine" by David Vizard. British cars use SU carbs (well some of them anyway), which are verrry similar to CV carbs. more simple actually than the bike carbs, but all the principals apply.
Wiredgeorge is right, intake tuning is done by adjusting the length of the velocity stack, or intake runners (cars). BUT, you don't necessarily have to have a long stack to get the benefits. You need to break that 90° edge where the air goes into the throat, it is possible to use a short stack (and they do have them, so short in fact they look like rounded rings). My MGB uses a 1/4" plate with a radius to help smooth the air flow.
The shape of the mouth of the velocity stack is verry verrrry important. You want a large smooth flair, so the air can flow smoothly into the carb, if the edge of the mouth is too sharp you don't get a good boundry layer, the air will not flow as smoothly, and you get turbulence. You want something that looks like these (this is not a recomendation for the manufactuer or product, just an example of shape)
www.velocity-of-sound.com/velocity_of_sound/velocity_stacks.htm
Not sure if any of this helps with the original question, but thought I'd toss in my pennies (better than tossing my cookies, eh?).