I long ago gave up on the case. I know one circular slot is for the collet not in use. The 1617 has a ton of accessories, you might look some up and see if they will fit.
Thank you. I'm getting the sense that the case is not particularly well regarded in the community. I like your solution to simply build a cabinet for the routers and their accessories.Welcome to the forum Riley. I have two of these and the cases are in storage. While nice they do take up a bit of room. I built a wall cabinet for my routers and keep them at hand there. The router table has 3 drawers and the accessories are in there which is far handier for me. One of the more important pieces is the centering cone, do make sure that's kept safe.
I’ve actually thrown away the majority of cases that I’ve had . Kept my Festool and my Bosch hammer drill cases thoughTool cases are sometimes helpful for the person who travels to do woodworking. Which may be why Festool sells their systainers and others their packouts, etc. I'm pretty sure the various slots, etc. are pretty generic for whatever you may want to store in the case. Like most have said, I don't use the cases at all.