I would make a 5 gallon batch of something and then sparge off a little extra wort for experimenting with the wild yeast culture.
I sort of did that today with my first attempt at a Berliner Weisse. I cultured Lactobacillus (hopefully) from some crushed pale malt and warm water (1 cup grain to 2 cups water, maintained at 95F for 24 hours). I actually made two of those cultures, just in case one went bad or something. I also made a standard DME starter and pitched the dregs of 6 bottles, 4 different sour ales, that I had in the fridge. I used a slow cooker base for heat, setting a cookie cooling rack on top and the cultures on the rack. I put a thermometer in one culture and it was a constant 95F with the slow cooker on the lowest setting. Kind of lucky
. There was some gas production, probably CO2, after about 10 or 12 hours. Lots of slow, bubbly rearranging of grains and stuff. When I checked it in the morning, it was covered with a white film and it smelled "lactic" to me. It smelled like sour beers. I figured it was doing the right science-y thing and decided I was going to brew.
I mashed for an hour at 150F with 5.75 gallons of water.
3lb Pilsner malt (appears to be fairly standard ingredient)
2lb Light Wheat Malt (standard ingredient)
1lb White Wheat Malt (sounded good from other recipe descriptions)
I also mash hopped with 7 grams of Northern Brewer (because that's what I had in the freezer).
After 5 minutes in the mash, it was only at about 147F. I decided it would be a good time to learn how to do a "decoction mash"... fancy. I drained a gallon or so of wort and boiled it, then poured it back in and stirred more. 150F.
60 min later, I dropped in my (sanitized) wort chiller. Yes, right in the cooler mash tun. Screw it. Then I cooled it down to 110F-ish.
I had a home depot bucket sitting around and decided it was my new sour beer fermenter. I fit a paint strainer bag in the bucket and drained the wort into it. When it started to get full, I dumped in both of my grain-based Lactobacillus starters, liquid, grains and all into the bucket too. I put the cover on and airlocked it up. I'll try to maintain 95F -ish temps by various ghetto methods for a few days or more. I figure I'll taste it in a couple days for sourness, then daily until it has a noticeable sourness. Then I'll pull out the strainer bag with all the grains and cool to 65F. Then I pitch in some basic yeast and ferment until gravity stabilizes. (I'll be using some yeast that I grew from a bomber of Green Flash West Coast IPA, because I'm a cheap bastard).
I also decided to get even crazier so I "cold sparged" with another gallon of tap water and lautered into a 1 gal jug. (poured a gallon of water on the grain to get more, weaker beer). I then dumped in my starter that I made from the 6 sour beers I had in the fridge. We'll see what happens. It was a bubbling starter. It should do something.