That's just it!, it does'nt have to be round. It seals just as well when the o~ring makes 90deg turns. Or do you mean the cross section?, cause that is usualy done 'square' as well
like this:
|_| not
U shaped, a U shape would prevent the o~ring expanding/compressing properly...
Can you do turns on the X~Y table?. Do you turn both hadles at once or something?...