I like privacy.com for things like this and you might also. You link your bank account and then you can set up on-the-fly credit cards with any name/address and arbitrary limits. You can set a maximum credit limit, or monthly recurring limits, or other configurations. It's also great for one time payments for shopping online.

I think they even have a rewards program now.

I highly recommend it if you can get over the uncomfortableness of linking a bank account to it. I just created a separate online account for this purpose.

In this case, I'd just create a separate card for each project you want to support with a monthly limit equal to your desired amount. They have an app and browser plugins and the like. It's really easy.