What about something like a tax rebate for high voter turnouts as percentage of the state's total eligible voter population? The amount would have to be large enough to be consequential. Making it based on a percentage of population would give small states a good chance. It might also end up being an extra pressure on states to open more polling stations nearer to voters. If a percentage of total eligible voter population is skewed too far in favor towards smaller states (easier to organize, less polling stations, or whatever else), then some combination of that plus total voter turnout might be more fair. This idea might also take advantage of the love that team sports receive.