Just let them go at each other with very limited restrictions. You need to have someone as a sort of referee to keep everyone from ganging up on one opponent. But thats all. A panel is too confining. They don't like that direct questioning.
Town meetings are also good, using ordinary voters to ask the questions.
Some reporters used to make fun of the questions voters would ask. But when they were given the chance, they asked good questions. One of the general election debates should be a town meeting. They ask stupid questions about policy. Voters don't want to know about that stuff.