Try the lampuhouse, it's the general go to place for overlanders in the Kao Sarn area, and has safe parking. A little more expensive than the others in the area, but also a little nicer. It fills up quickly though, so advance booking may be necessary.
However, if you're just around for 2 days, don't have a bike with you, and don't have special whishes, I don't think you need to reserve anything. Just hop on to the tourist bus from Suvannabum to Kao Sarn and you'll find plenty of hostels for all kind of tastes and wallets. Personally I don't like the Kao Sarn area, and would recommend to leave Bangkok straight away, but if you absolutely must stay for whatever reason, it tends to be the easiest option.