Hi Dug,
Bunnings has a good range of wall vents for sale, you will be able to pick out something with a nice finish to match the units.
How interested are you in doing a bit of research about thermodynamics? The placement of your vents in relation to your heat source has a great impact on air flow within the enclosure. Also the size of your vents are dictated by the volume of your enclosure. I would suggest buying vents like this, there is no chance of escape through them and you can vary the amount of air flow through them. To vary the amount of air flow, tape up sections of the vent (on the outside). Make the taping matches on both vents.
This is how my enclosures are set up.
There is a vent near my heat source. The hot air around the heat source fills up the space under the hide box. When the space under the hide box fills up with hot air, some of the hot air escapes through the vent, causing cool air to be drawn from the opposite vent. This process of heat escaping and cool air being drawn in creates air flow.
The air being drawn in needs to be cooler than the air within the enclosure.
You will know when the system is working when you can feel the air being drawn from the 'cool' vent.
Best of luck with the project.
Nick