After a few hundred mile ride the other day, the setup was MOSTLY okay, except towards the end, I stood up on the pegs to stretch my legs, and in doing so pushed on the backrest, popping the seat up and out.
Not a fun situation.
So the next chance I had, I went to the garage with a Dremel and decided to either fix the seat or destroy it, whichever came first.
I'm happy to say I fixed it. It seems the Hartco is made mainly for a standard, the taller tank of an Interstate caused it issues. I Dremel'ed away some fiberglass from the nose of the seat, and it now latches into place. I've sent my info to Hartco so he can update anyone else that might have this issue. I'm pointing to the area I removed material in the pic, just for posterity:

Also, I've got a taller windshield now, with a vent, and am loving it!

Now that I've got the Interstate set up juuuuuuuuust right, I need to remember to take my Standard out for a spin soon, lest she get jealous and lonely!