Welcome aboard, joe527!
When I had an Ultra slug 12ga, I cast the Lyman 525gr hourglass slug from wheelweights and handloaded them using a LEE LoadAll II. These load as a sabot using regular shotgun wads and a fold crimp. Not counting equipment outlay, cost per slug was about $0.59!
These were as accurate as could be hoped for, and cut quite a huge hole in a deer.
This mold does come in 20ga. If you already do any casting, you may want look into this.
Also, Ballistic products has some slug offerings. Take a look.
www.balisticproducts.comI found this slug to be far too brutal to shoot from the Tracker II 12ga that I have now (weighs 5# instead of 9#) , in the loads that produced the best accuracy, and have since gone to a storebought sabot.