It's good to know the methodology thanks JanSteen, the same applys to agar, unless one uses gentamicin, rationally it still gives fungi a chance for growth beyond the perimeters of contaminates... I work in a shrumovbox so relative sterile work won't be hard to perform
"It's not the resistance of cardboard, it's just that most bacteria from human source can't break down cellulose. . . "
Now that can be a problem LOL. Judging from your pic's still looks feasible to isolate a suitable sector.
cardboard soaked in water + fungus that secretes cellulase enzyme, doesn't need much shaking after just a few hours at optimum temperature. The cardboard would just degrade pretty fast, within hours maybe.