I have a space around the bottom of my Mattone Cupola

Before seating my Mattone Cupola oven to the hearth, I put high-temp mortar under it. When I removed the blocks to seat the oven (literally about 5 min after mixing the mortar with cold water), the mortar did not squish down very much and now the oven is sitting about 1/2" above the hearth on top of the bead of mortar. The mortar must have started curing very fast. Should I use castable refractory or high temp mortar on the inside and outside edges of the base of the dome before covering?

Other posts mentioned Rutland fireplace mortar (Amazon.com : 10.3 rutland fireplace mortar), but the instructions specifically say it’s for indoor use only. If i’m covering the oven with stucco, will that be enough protection from water erosion?