Are there any animal shelters around (preferably north) ATX where I can volunteer without a pre-set schedule?

I know this is a strange request, but I’ve recently started to look into volunteering at animal shelters on my off days. I volunteered a lot in my home town and the process was simply calling the shelter to ask if they needed help that day, then showing up if they did. Here, however, all the shelters I’ve checked out require volunteers to make schedules and commit to specific hours each week. I 100% understand that it’s probably safer and more reliable for the shelters, but as a poor person with a constantly shifting schedule, I unfortunately wouldn’t be able to commit specific times each week to volunteer work. If anyone knows of any shelters where I don’t have to pre-plan my times and can just volunteer whenever I find a spare few hours, I’d really appreciate it!