Landscape fabric would probably work, although I have the litter bags on hand already. The soil under the beds is pretty decent (used to be farm land) so letting stuff shoot roots down should be fine. I’m just trying to do just enough to keep soil from spilling out between boards when it rains.

Also, turns out my city has a partnership with a place that sells compost from the yard waste the city collects so it isn’t completely insane to fill the beds with compost. I’ll probably do some more poking around to see if I can find anything else though.