If your case isn't designed for them, then they won't do you any good. Sell your case and get one that comes with them. Kingwin, Silverstone, Athenatech, Evercase, etc., etc. sell cases with screwless drive mounts.
. You could use thumbscrews as they are available in the M3 size that optical drives use as well as the #6 that hard drives use - but you may have to shorten the threaded end a bit or use washers as you want the threaded part to intrude as little as possible into the inside of the drives. I use a toothpick to test the available depth on my drives.
.bh.