I found a site with some cub cadet service manuals. It appears to include the SLT models.
https://www.ccpartsnmore.com/cub-cadet-service-manuals
I tried to upload a picture of my wing nut plug, but the photo is way too large. Have to ask my son how to compress or leave the photo somewhere as a link.
Just a quick comment on the wing nut plug, it will not fully seat as the wings will hit a part of the trans so you will have to shorten it or grind a part of the wing to allow it to seat. Photo will show as soon as i can post. I like the wing nut because I can remove with my hand in less than a minute and add fluid until it over flows but cannot check the level.