It looks like my little experiment in seed sowing with toilet paper rolls might be coming to an early end as I spotted fungal mycelia this morning in several cells and on the side of some toilet paper rolls.
Strangely enough, the one plastic 4-cell pack that I did sow is fungus-free.
I think this has to do with the fact that I thoroughly disinfected the plastic pots by soaking in 10% bleach, whereas the toilet paper rolls were not disinfected at all. Also, I think that the toilet paper rolls might still have been too wet even after drying overnight. Instead of bottom-watering the filled TP rolls before sowing, I should have probably dampened the medium, let it dry to an acceptable level of moistness, then add it to the container, then sown. Live and learn.
I'm going to try to save the stuff I've sown in toilet paper rolls by spraying with chamomile tea and sprinkling with cinnamon. In the meantime, I will be moving the rolls out of the germination and propagation station and onto the top of my fridge in an attempt to save the rest of the plants (including the lithops seedlings!) from collateral damage.