Alexis, I made this with scrap wood.
The "box" was made to cradle the log all the way around. That way I did not have to drive screws or nails into the log or use wedges to hold it in place.
The "slide" was made from the same scrap wood. As it turned out it was almost too thick for me to use. Between the thickness of the wood slide and the router base, I used every inch of the router bit's length and the max depth of the router's shaft to reach my desired depth of cut into the log. I even took the router's base plate off and mounted the router to the sled for an extra 1/8" depth of cut. Working at the extreme limits like this is not the smartest thing to do.....but I got away with it this time. I will make me a new slide before using it again......a thinner set so I don't have to push the limits of safety next time.
I installed slide stops under the slide so I would not cut into my case. Using the outside edges of the slide I was able to maneuver the router back and forth to cut the log. But, when it came time for the precise, close cuts, I still held on to the router.....didn't trust myself fully just yet.
I took my time to make the jig with smooth, rounded edges. Be sure and make the slot in your sled wide enough for you to see what's happening with the cutter and to allow chip clearance. I used screws so I could take it all apart and make dimensional changes for the next project I will use it on.
I really like working with logs. Using this method to hold the logs in place makes this part of my jig safe for me to work. It's probably not the most technical system, but it works for me.
When you are designing one for yourself, just take your time and think about every way you can get hurt trying to hold down a piece of material. There are safe ways; there are quick ways; but there are few ways that are quick and safe. I trust my design.....but that doesn't mean it's the safest method out there. So use a lot of common sense when you make one.
Thanks for asking.....and post pictures of yours!!
I may not know what I'm doing, but I am having fun doing it wrong!!
Happy Day Veterans!!