Having recently done this myself for an oversized hanging closet door it's actually easy and cheap. (with nice results) I bought 2 sheets of hardboard .115 (@ $8/each) and poplar (or pine) 1 3/8 in 8' lengths. These were $1.80/each and you need 5/door depending on width. Cut 2 of them to the width of the door and the remaining 3 to the desired height of the door (subtracting 2 3/4 since the top and bottom pieces sit are respectively above and below the vertical members) Build the rectangular frame from the 5 pieces of 1 3/8x 1 3/8. I used finish nails and glue for strength and laid them out on the flat floor of the garage. Once they are dried run glue along the face of the frame and lay the hardboard on top (mine overlapped because my door was smaller than 4x8' that the hardboard comes in) Once the glue dries on that side use a router with a straight bit with a roller on the bottom to flush trim the hardboard even with the frame. Flip it over and repeat the glue up and when dry the flush trim. The hardboard has a very fine smooth finish that takes paint well and this makes a lightweight and cheap door that is very strong for it's weight. Depending on the width you may need a 4th vertical as the hardboard is quite thin and you don't want it to sag. My closet doors cost about $26 each + paint and hardware. Hanging hardware for my roller doors was ~$7.00 for 2 rollers on top and a plastic guide for the floor. I couldn't find anything that even came close to that price in a custom size and they look great.