Well you could easily and safely allow 25% capacity, spread out the folks with empty rows and seats in between (and I don’t mean everyone is every other seat, rather groups (like a family) can sit together but apart from other groups, and require that everyone must wear a mask. If you get caught without a mask, you get a single warning, a 2nd violation and you are escorted out of the stadium.
I’m not saying it should be done like this, but since you said “don’t know how else to do it”, I thought I’d give an alternative option.