Here's how to get the Si5351A kit very stable indeed, even without using the OCXO version. This is the work of Jon Ove LA3JJ. But I know of several people who have done similar things, with similar results (myself included). So it certainly works. With these changes, Jon Ove showed how to get 0Hz drift reports on the 10m band (28MHz). Impressive frequency stability!
Three things combine to get this good frequency stability performance. All are important. The combination of all of them produces the excellent result.
1) Use the "park modes" in the U3S kit. What they do, is allow the Si5351A Clk1 output to be configured to produce a frequency during key-up. The Clk1 output is not connected to anything in the U3S. But setting up the Si5351A to produce a harmless frequency on Clk1 during key-up, keeps the chip "warm" during key up, which greatly reduces the drift on key-down. The internal chip temperature seems to affect oscillator frequency. Which is not really surprising, as all the oscillator components except the quartz crystal itself, are inside the Si5351A chip. Jon Ove used Park Mode 2, Park Freq 149MHz. (I used 150MHz, this is nearly identical as far as keeping the Si5351A "warm" is concerned).
2) Put a heat sink on the 27MHz reference crystal. Jon Ove used a metal hex spacer, on its side, glued to the top of the crystal. I used a small TO-220 style heatsink, strapped on to the crystal with a rubber band. Basically you want something metal to increase the heat capacity of the crystal, and slow down any temperature changes.
3) Add some insulation to keep air currents way from the crystal, again to keep temperature variations away from it. Jon Ove carved out a piece of foam to fit over the top of the Si5351A board. That's better than I did, I just installed my U3S inside the standard U3 case to keep air currents away. For sure, Jon Ove's way of insulating it properly is nicer than mine.
The images below show a photograph of Jon Ove's modified Si5351A Synth, and the results on WSPR showing 0Hz drift on 10m (local copy). Click the photos for full size versions.