I went to the local electrical wholesaler and tried to buy Dux Seal. He said no one uses that anymore. He advised me to use expanding foam or silicone caulk to seal the conduit to the service panel. I think the Dux Seal advice given me here is better advice. Expanding foam could be difficult to remove and silicone caulk contains acetic acid, at least while curing and possibly conductive. Also, using a metal caulking gun inside a live electrical panel might be hazardous to my health. I think he simply doesn't have it in stock. I'll keep looking for the Dux Seal.
Bob