Love the cardboard idea. Sounds like it would work fine for oysters, shitake, & other wood loving species.
I was wondering about Stamets recommending changing agar. Is it really necessary, or just another way Stamets can promote selling more agar on his site.
Let's examine the steps in mushroom cultivation and think about it.
Stock cultures are maintained on agar plates or slants. Most "all purpose" media contain simple sugars as a carbohydrate, & maybe peptone or other nitrogen source. PDA will have some starch from the potato infusion, otherwise there aren't a lot of complex carbohydrates.
Mycellium is transfered to grain which will be used as spawn. Grain has to be malted & mashed for starch to be converted to sugar. Whole rye isn't malted, so you've now gone from a sugar based media to a starch based one. There is a lag phase as the mycellium adjusts to it's new environment, then healthy growth.
The next step is the spawning of a bulk substrate. Depending on the species of mushroom, the substrate may be a dung & straw based compost, maybe a wood based one. Again there is a lag, then growth & hopefully fruit.
The different nutrient sources all require different enzymes for utilization. The ability of an organism to produce various enzymes is genetically predetermined. I'd have to disagree with TMC the ability to digest certain food sources isn't lost just because it wasn't there in previous stages of growth. Going from agar to rye to compost doesn't seem to slow these guys down.
How about formulating you own media for culture maintenance. If you made one that had a similar nutrient makup to your spawn or sub, there would be the least shock to the organism when transfered to a new medium. Try using water from rye cooking, or compost pasteurization for your next batch of agar, pass it through a coffee filter & prepare as usual.
Check out
Fundamental Principles of Bacteriology by A.J. Salle. There is a good chapter on pure culture techniques.
_________________
