If you're familiar with muscadines, I think you can imagine the force needed to pop their skins on a bucket bottom sized area even if it was only 2 layers of grapes. I think the bottle bottom sized area popping 4 or 5 grapes at a time might be more feasible. I used a food grinder once. It was one of those 60 year old galvanized cast iron things you clamp onto the edge of a table. There is a straining disk with bb sized holes, more or less that you remove. Also there is a cutting blade like a dutch windmill that you remove so it's just an auger pushing the grapes through half inch size triangle holes. This won't break many seeds if any. But chewing up the skins probably releases a lot of tannins and harsh tasting stuff you might not want. The batch I did that way was harsh (I didn't age it long enough either) but some of the harshness could have been due to the 16 to 18 percent alcohol. I've crushed them by just popping the skins in my hands, but that's a lot of work unless you're just doing a gallon or two.
Some people have taken a 4x4 piece of lumber, wrapped the bottom in Saran wrap and used that in a bucket to crush them. I can imagine the things slipping out from under the slick wrap, so if you had lumber that you know has not been treated, the rough end of the wood would tend to grab and hold them in place. Just be sure the wood is sanitized with K-meta.