Years ago I converted a bunch of Corney Kegs to store syrup. Based on my experience I suggest you weld a flat plate over the oval top opening. Before that, cut out an opening the right size to weld a 1.5 or 2" sanitary over the hole and remove the spring loaded blow off (vent) and weld it shut. Then use a 1.5" or 2" sanitary plug , silicone seal and clamp. If you can't weld SS find a good welder who can.
My method was much cruder and failed about 10-15% of the time. I made a way to push the top down tighter, but some O ring gaskets still leaked. I even tried one year filling the kegs with hot syrup, then I pumped food grade nitrogen in to 35 psi using the original closure. Some still failed. I see welding as the best method. This is where I buy my sanitary fittings and adapters https://www.brewershardware.com/Sanitary-Fittings/
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.