A slug design I have liked and thought about is the Balle Sauvestre:
http://www.sauvestre.com/12-70,1381?lang=fr
Pretty much out of the home tinkerer's domain... until the advent of 3D printing.
Originally I liked the idea of the BRI sabot slug but they reportedly didn't stabilize from smoothbores well. Taofledermaus "proved them wrong" at least in their videos. My thought was to add a tailwad but Balle Sauvestre beat me to that! They have a pretty slick idea that apparently works well.
I have not tried either slug partly because they are hard to get where I am and partly because I am too cheap to pay for factory slugs and because I want to be able to do it myself. Also, I prefer bore size or near bore size slugs rather than a .50 or .60 cal. sabot slug. 12 ga. shotguns are big bore so let's shoot big bore slugs!
Now with 3D printing available some of these things are doable at home for those with 3D printers... you being one of those guys!
Not sure if you have seen BigMrTong's threads:
http://castboolits.gunloads.com/show...in-12g-Shotgun
http://castboolits.gunloads.com/show...stom-Slug-Wads
http://castboolits.gunloads.com/show...-Lee-Slug-Load
http://castboolits.gunloads.com/show...-Slug-Accuracy
He has played with Lee slugs and some home designed slugs using tail wads and sabots all 3D printed.
With the ability to 3D print I'd lean towards a full bore slug with tail wad or a Lightfield type sabot slug near full bore with tailwad. That's just me of course.
I'llbe watching to see what you come up with and field results. It is indeed fun playing with slugs!
Longbow