I use appliance Designer for all of our splint designs. It basically does what the C&B module "Splint Designer" can do but the appliance designer is like the sandbox version where you have control over your workflow and can change stuff at will.
I have used it for the past few years for all our designs and its allowed more control over the process using the appliance designer over the set workflow. There is a bit more of a learning curve because there is no wizard, but you can create one once you have the process you like down pat.
When I design, I use a 0.1 mm offset for the internal shell. I do a block out (Remove undercut) first based on path of insertion and a default value of 0.5 mm retention. I usually need to add additional block out in the inter-proximal area as the remove undercut will not add any here.
For our splints right now we are printing 2 variations, one is in essence a mold of the splint so we can use our flow-able acrylic that we prefer, and in other cases we are direct printing the splints in our Form Labs. Keep in mind however that when printing or molding the material will play a big part in your retention and offset settings. The Form Labs material is very rigid and we switch to a 0 mm, 3 degree block out and manual addition with the same internal offset.
You will need to find the right settings for the material you are using, the more rigid the material the less retention you want to add in the settings for "remove undercuts".