I just replaced the apron on my AVL. Went to the fabric store, found cloth slightly narrower than the beam and some fusible interfacing. After fusing the interfacing to one end, I sewed the cloth back on itself to make a channel for the rod and cut 'tabs' I reinforced the edges of the tabs with fray check.
I attached it to the beam and inserted the apron rod.
The beam on my AVL is metal, but if yours is wood you can staple as above or use small upholstery tacks.
Maybe try back to front next time a compound sleying is required?
It is a good idea to know several different techniques because no one method will work well in every situation. :-/
If you haven't finished beaming, try applying tension to the warp. That will sometimes help. But yes, 3 in a dent FTB will pretty much be a problem in my experience. That is the primary reason why I stopped beaming FTB
I was trying to beam 2/20 mercerized cotton, three per dent in a ten dent reed and it was a nightmare. :(
Hope you can get the warp dressed so you can get to the shuttle throwing...