2. What tools are required to build your own inserts? I presume that a
planar/jointer would be required to acquire the correct depth. I'd
hate to go this route because of physical space concerns.

All you need, really, is a router with a template bit and some good plywood,
such as 1/2" baltic birch. You can even use MDF. Forget about any
planer/jointer requirements and use sunken allen head set screws to set the
depth. Just cut a blank slightly oversize, attach to your existing insert
with double-faced carpet tape, and use the template bit to make an exact
copy. If 1/2" is too thick you can rout a rabbet on the underside.