There are several options for alternate accommodations should the site be full or you would rather stay on your own.
The closes alternative accommodation is in the city of Alingsås. There you can find the following hotels.
Grand Hotel Alingsås
Lilla Hotellet i Alingsås
Booking and payments for the off site accommodation is done directly with the place you are staying. See their websites for more info. If you choose to stay off site you still need to pay for the events Day pass.
Other booking sites may also have some accommodations close by.