For sealing of the electrical feedthroughs, we would recommend using polymer solutions, for example, a normal lab binder used for shaping powders. The polymer should be stable up to 75^oC or 165^oC for a high-temperature base unit. In our lab we use a mix of  B60 and B72 paraloids dissolved in ethyl acetate. For sealing the base unit should be fixed upside-down, and a few drops of binder should be added in each feedthrough hole. Preferably, the cell should be simultaneously evacuated. A transportation quartz tube can be used for locking the cell during sealing. Alternatively, a leaky feedthrough can be sealed from top using a high-temperature vacuum tight glue, like JB Weld.