BLUE TONGUE SKINKS

Does anyone have any recommendations for making your own blend of substrate that will hold tunnels?

I looked at the Excavator Clay substrate commercially sold, but I disliked that the tunnels would harden.

I've read of people adding organic potting soil -- but what organic potting soil? I couldn't find any that didn't include some kind of organic fertilizer, which I assume we'd want to avoid.

I usually use a blend of cypress mulch and coconut husk. Our Bluey can dig, but it doesn't hold the tunnels. I'd like to add a substrate that can hold tunnels for some added fun and stimulation for our Bluey!

Additional Question:

While I'm at it, does anyone know if Zoo Med's ReptiFresh would be good for Blue Tongue Skinks?

Soil sand mix and pack it down really well should hold tunnels. There’s a lot of mixed opinions with organic soil, but I’ve seen a lot of people use ones with organic fertilisers no problem and that you just need to avoid ones with manure or chemical ferts.
I personally just pile some sugar cane mulch in a corner or in a repurposed cat litter tray for mine to make tunnels into and they seem to enjoy it.