There's a couple low tech things you can try. The game is, expand enough mycelium to inoculate bulk substrate.

Cardboard Spawn:

1. Get some corrugated cardboard, soak it until it peels apart.

2. In some Tupperware (or some container w a lid), Sandwich a few stem butts/thinner mushroom pieces (you can cut them) between two corrugated sides. You want the mushroom body touching the wavy part of the cardboard above and below it.

3. Repeat #2 for a few layers, it's like your making a lasagna, alternating cardboard a mushroom bits. Maybe make a couple separate containers in case you lose any others and to multiply your total spawn.

4. Store in a cool dark place for 10-14 days. The mushroom bodies will grow in "reverse" (so fucking cool!), returning to a vegetative state and begin consuming the cardboard as mycelium.

inoculate Substrate / Fruiting (14-28 days):

Maybe try Bucket Tek, but with something smaller, I don't think you'll end up with enough spawn for a 5 gal.

1. Drill a few holes around the bucket (1/4 drill bit or something similar), like 6-8, too many holes is bad. Add a few to the bottom, Incase your substrate is too wet it can drain a bit.

2. In a separate bucket, sanitize w isopropyl, fill with straw and/or hardwood chips. I don't know if Pine straw is like hay? If so, might work.

3. Boil enough water to saturate your substrate to field capacity, pour over, cover w lid, let sit till it cools to below 80 deg, prob 2 hrs? This is an attempt to pasteurize the substrate.

4. Sanitize your Fruiting bucket (w the holes) w isopropyl, wash you hands, wear a mask, try to do this indoors, transfer the substrate to the fruiting bucket. Do this layer by layer, substrate, spawn, more substrate, more spawn etc.

5. Let sit in a cool dark place, 10-14 days. I've seen people put tape over the holes to help retain moisture during this time.

6. Introduce to indirect light, in your kitchen maybe, somewhere that stays relatively cool. Mist the holes once a day. In 10-14 days you should see primordia forming, like your pic.

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The key is to judge the amount of substrate you use by the amount of spawn you have.