Boudin can be made with basically any meat or seafood. Crawfish are as good as pork in my opinion. So use what you have in your freezer or fridge and have fun with it. If you don't want to make cased boudin, roll it into balls, bread it and fry it for the ultimate Cajun party treat. My recipe below is an amalgam of what I saw at Legnon's, from Chef Donald Link's book Real Cajun and from former Tabasco cook Eula Mae Dore's book Eula Mae's Cajun Kitchen.
Chop the meats, liver and fat into chunks that will fit in the grinder. Mix the meats, liver and fat with the onion, celery, poblano peppers and garlic, then the salt, curing salt (if using) and either the Cajun seasonings or the spice mix you made from this recipe. Put it all in a lidded container and set in the fridge at least an hour, and up to a day.
Put the contents of the container into a large pot and pour in enough water to cover everything by an inch or two. Bring to a simmer and cook gently until everything is tender, at least 90 minutes and up to 3 hours. Strain the cooking liquid (you'll need it later) and spread the meat, fat and veggies out on a sheet pan to cool.
When everything is cool enough to handle, grind it through the coarse die (6.5 mm) on your grinder. You can also hand chop everything.
Put your meat mix into a large bowl and add the cooked rice, parsley and green onions. Mix well, and add up to 4 cups of the reserved cooking liquid. Mix this for 3 to 5 minutes so you make a more cohesive mixture to stuff into a casing. You now have boudin.
You can just shape the mixture into balls and fry them (they're awesome), or use your boudin as stuffing for something else, like a turkey. Or you can case it. Stuff the boudin into hog casings, and while you're doing it, get a large pot of salted water hot -- not simmering, just steaming. You want the water to be about 165ºF to 170ºF. Poach the links for 10 minutes, then serve. If you are not serving them right away, no need to poach the links yet.
Boudin does not keep well, so eat it all within a couple days. It does freeze reasonably well, however.
Note that while I poach my boudin, the links are also excellent grilled or smoked. If you smoke them, you absolutely need to use the curing salt, Instacure No. 1, which you can buy online or in some butcher shops.