On an evaporator it is very uncommon to have a cap when boiling. Some use a flip open cap so it can be closed when not boiling.
If you still get smoke, you may just need more height. Yes, 2x the evaporator length is the guideline, but several things can affect what you need. The surrounding hills, buildings, trees and other things often make more height necessary. The space near the back of your evaporator (inside the arch) can also affect it. I suggest you just add 2' more, using single wall pipe on top of what you have, if that doesn't correct it, add 2' more.
Also, you say smoke, if you get smoke out once you have a good fire going, your wood is either wet, not split small enough or punky or for some other reason it is not burning hot enough. On my evaporator after the initial lighting you can not see any smoke, it is clear once the fire gets going hard. Once going hard, even punky wood, if dry will not smoke.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.