If it is awkward to make up a tempo to fit the amount of time you have it mind, you might be able to use "set measure/beat at now" to let Sonar calculate the fake tempo to match your idea of when the next measure should start.
Briefly, move your Now time cursor to a point in
time (ignoring any midi notes that are already occupying that time) where you would like the tempo to resume, then use "Project -> Set Measure/Beat at Now" entering the measure number, beat 1, of the measure in which tempo should resume. A fake tempo is created and the
a tempo measure(s) will now occupy the time you had in mind. But measures to the right will all move and change length to match the newly calculated fake tempo (just as they would if you had entered the fake tempo yourself). So then change the tempo at that chosen measure to resume the nominal tempo of your piece. Later, you can still make slight adjustments to the
a tempo measure(s) by editing that fake tempo. It would be a nuisance if you also wanted to change the number of beats in those measures.