https://easyfirepits.com/
I converted our our outdoor wood fire pit table to a propane table about 3 years ago using a kit from easyfirepits.com. They have an excellent web site and claim you can turn almost anything into a fire feature. For $250 +/- it came with everything except the fire glass beads. Very easy to set up and we use it almost every night. Hours or maximum enjoyment from minimum cost. They don't throw a ton of heat (think about your gas grill with the cover open) but the ambiance is awesome. We also added a glass wind block surround available from amazon in almost any size.
You could try using a removable fine mesh window screen over the gas fire pit to prevent marshmallows from falling in.