How about cactus? it's technically designed for comments on a blog (I'm using it on escapebigtech.info for example), but it allows you to use any matrix room and if you set it up to use a different matrix room every time it loads the page it will work perfectly for your usecase.
Please keep in mind that if you decide to adopt it, Cactus can only exist if you help it exist, if you can afford it and you get value from it, I encourage you to support the developers through donations. Embrace the Value4Value principle!