If you don't hold the o-ring with a three sided groove, then when you compress it, as you screw down the top, the o-ring will compress into the block, instead of compressing up against the top.
This is a surefire way of having a leak.
I'll be using a 1/16" o-ring, which is going to be more than plenty for the job, but the o-ring groove has to be perfectly smooth, and the o-ring has to sit in the very middle of it.
I had a very difficult time with my block and the o-ring, because I didn't have a lot of room for that groove. On the plus side, I'm bolting the top, with no less than 8 screws
(six would have done fine, but my design can't handle 6 bolts!)