o/
If you’re willing to change up the design aesthetic and have other materials lying around (e.g. dowels, rods, some kind of sheeting), you could also just print the “end caps” with slots for the main body.
This also has the advantage of being sturdier and maybe easier to print, but the design might be more involved.
speculating here, but the “auto-generated ones” are probably just the ones in the opengraph tags, which is supposed to be what the website intended as the thumbnail. It’s more likely that these websites’ operators don’t have a clue that opengraph is being used, but since they’re the “proper” way to do embeds, lemmy can’t just ignore them, either.
or even a center strip that “staples” the two halves together with pegs running through holes in the two halves (probably sturdier than glue but also uglier)