I used this combo:

Topeak MTX

and

Molle Pouch

These pouches come conveniently with the rounded strap which loops thru the seat post. It can be velcro'd to the rack and/or the Topeak. So it ain't going anywhere. Hope you can find a Topeak - I paid about $52 for one. I did add another cross bar to the rack if you notice (if Aventon rack) on the inside bar there's already a starter hole so I cut another metal crossbar then used self tapping screws to tie it in. Secured with zipties I suppose welding it is better but, it hasn't moved. A better location to tie in the Topeak since I'm pushing it back on the rack to fit the Molle pouch. Both both fit on the rack.