I wanted to make something similar myself, but couldn't find any lace that was wide enough.
After a lot of searching, I remembered that I CAN CROCHET- and my problem was solved! )))
I found great diagrams here, tried couple of them and found the one that worked for me!
I cut the slit a little bit smaller than the lace ,sewed it to the shorts and VIOLA!!!
I am in love with my new shorts!)))
Thanks for stopping by!