Considering Google, it gives NO keyword credit to meta keywords tag. But if you want it for other search engines, then I guess a good way is to put your main keyword, then generate long tail keyword phrase from your main. What you stated can work out, such as this:
HTML Code:
<meta name="keywords" content="garage conversion,garage conversion in edinburgh,edinburgh garage conversion,garage conversion in glasgow,glasgow garage conversion,garage conversion in stirling,stirling garage conversion" />