Even in an open case you might want some airflow over the cards, although if they're 750tis then they run pretty cool anyway.

I built an open case using this site as a guide: http://wilsafris.wordpress.com/

It cost less than £20 all in and was dead easy to do.  I do have a desk fan blowing across it to keep the cards cool however they are R9 290s so run a lot hotter than 750tis.