I've used a stick when I got off hunting and found I had left the mag 'plug' at home.
Apparently, the 320 is a copy of the Winchester M1300. So anything for a 1300 will fit a 320.
Choate makes a mag extension for a 320. Only adds 2(that'll give you 8 with 1 in the chamber) and fits an 18.5" barrel. Doesn't come with a follower. $52.03.
https://www.riflestock.com/store/pro...xtension-matt/
These guys list an 8 round extension for a Win M1300 for slightly less money.
https://www.midwestgunworks.com/winchester-1200-1300/parts.html#/filter:hierarchy:Gun$2520Parts$253EWinchester$253E Shotgun$2520Parts$253EWinchester$2520Model$2520120 $252C$25201200$2520$2526$25201300$253EMagazine$252 0Tube
Brownell's doesn't list aftermarket stuff for Stevens or Savage's.
Spelling and grammar count!