"Best" is what works for you. There are a number of stands, as Dusty mentions. I found a waiter's tray table works 'best' for me. There are several height sizes available, you can usually find them locally at restaurant supply houses, and they don't have premium prices usually associated with niche goods.
As far as securing the dulcimer, it will depend on the stand, how firmly it holds the instrument in place, and how 'active' you are while playing. Some folks need a small bungee or strap or two; some just need a rubber pad. I don't need anything to hold different dulcimers in place; in fact I often have two on the stand for a 'hot swap' of different tunings.