You could always have your online RSVP be a ticket sales site. In order to RSVP yes, they have to buy a ticket with a respectable per person contribution
Do not do a registry my brother and his wife did not have a registry and there was a table for cards. I do not think it is tacky there are many people that do. I think if you want to do it do it! Find a way that you can create a PayPal. Or create a beautiful card with your photo and tell them you prefer money instead of gifts. There are so many people that do.
https://www.honeyfund.com/how-to-ask-for-cash (this I would use MyRegistry.com they do not take out a big percent of money out of the money you make and you are to custom make your fund.)
Sorry, I had to take a moment after reading all of these comments from people who think I'm asking for cute ways to ask for money... c'mon people.... just read the comments OR better yet-- get educated on your memes