I can't tell you what works, but I can tell you what to avoid.
I tried a Vortex Sparc on one, and a Burris Fastfire. The Burris wasn't that great. At that distance from your eye, if you're not lined up perfectly behind the sight picture, you won't see that dot... It's just too far. I found the Vortex Sparc a lot easier for lining up, but the magnifier on it sucks (the field of view is super narrow and it's eye relief is crazy short). It's all right, but want low magnification (like x3 or x4) in addition to 1x mag... So the Sparc doesn't fit the bill.
So I mounted an XS sight rail on mine, and I'm saving up for one of the tactical 1-4 or 1-6 scopes with a traditional (shorter) length of eye relief.



























