This Blackburn -- which seems to have doubled in price since I bought mine -- fit my Aventure well enough once I used p-clamps to attach its arms...
but... Just like the official racks from Aventon, the friction fit where the arms meet the rack itself is just plain bad design. It is a point of failure.
The "add some washers' fix?
Still fails because it's a friction fit. This idea that a bolt tightened down is going to hold any real weight is a joke. Especially if you try to put something like your spare battery on it. That fix worked for a friend of mine for about two months before it failed.
Thus with that blackburn rack, I ended up going to the hardware store. The blackburn's eye-hooks for connecting to the frame are 1/4" threaded 20tpi. So I got two threaded rods, two turnbuckles to adjust the length, and some bolts, couplers, and p-clamps (well, I already had the clamps) to connect the rods to the rack.
Which is far more solid and stable than any of the stuff that comes stock on most rear racks for bicycles. Irregardless of what rack you end up with, I'd plan to have to make "improvements' like this to it.