If you are going to shoot slugs occasionally, get a cylinder bore choke tube. Shoot rifled slugs only (like Brenneke's). Their own rifling imparts the spin that stabilizes them.
If you plan to shoot a lot of slugs, get a slug barrel with rifling, and rifle-style sights. Should run $100-$200. The adjustable rifle sights will allow you to adjust your point-of-aim to point-of-impact. You can't do that with a single bead on a smoothbore barrel. And the rifled barrel allows you to shoot saboted slugs, which gives you more versatility in loads.
Then just put your smoothbore barrel back on when you want to shoot clays or birds.
If you have not changed barrels before, it takes less than 30 seconds.