Step One: I simply glued 1x6 tongue and groove pine, cut size to cover my opening. I ripped the tongue or groove off the end pieces with a table saw for a smooth side, but if you don't have access to one you could just as easily leave it. For added holding power you could also nail through the tongue and groove joints with finish nails. I didn't have finish nails short enough and this is an interior door so I wasn't too concerned.
Second Step: Nailed and glued a 1x6 pine board across the top and bottom, lining it up with the end and sides of my t & g. Nailed and glued 1x4 pine side rails and a cross brace. I let it dry over night.
Decorative sliding door hardware was ringing in at around $300 for this 5'-0"x 7'-0" door. That just wasn't in the stars. We found this seriously meaty sliding door hardware meant for real barn doors at our local farm store for $50.00. We debated building a box to hide it on the exterior, but decided to just mount the door on the inside and hide the hardware in our unfinished utility room.
Total Budget Breakdown:
Tongue and Groove: $65.00
Pine Trim Boards- $20.00
Sliding Rail: $25.00
Sliding Wheels: $25.00 for two
I also built a simpler surface hinged version, out of leftover wood for the odd shaped opening to our under stair closet. I left off the side rails and the door is still very sturdy. I made sure to nail the ends and cross brace into each piece of t & g. The two doors face each other in a little sitting/ circulation area in our newly finished basement.