Thanks for the quick replies both!
@theFlu Hmmm, I tried figuring out what you meant, but the whole structure is symmetrical as far as I can see.
@8_Hussars I tried your solution (see first image below, arch at the back) and it works (although the blocks don't seem to match up perfectly if you look closely where my crosshair is, but I'll take it 
I think I might not have phrased my question very well. What I aimed to do is to continue it going horizontal (see second image below where I continued it with regular blocks in blue, just to show what I want) before finishing the arch on the other side, but no block seems to fit for this... any ideas?
Also weird that there seems to be 2 different ways to continue the arch: my first one and and the one 8_Hussars suggested.
