Do speculative applications work?

I’m a second year law student and haven’t managed to secure a single vacation scheme for the summer despite having excellent grades. Do you think it’s worth making speculative applications to smaller law firms (not high street as I’ve already done a stint at one). If so, what approach shall I take? Do I just pick up the phone or should I send them my CV and a cover letter? Also, what should I include in my cover letter?