Closing in a hoophouse
This is the 2nd hoophouse that I've constructed, and in the last one I used a plywood and 2x4 approach, covered with OSB on both ends. It was relatively quick, has stood up to the weather for 2 years, and allows me to cut ventilation holes as I need, I didn't like that it shaded the greenhouse in the morning and evening. It also is fairly heavy, and relatively expensive.
So for this hoophouse I've chosen to make a lightweight, transparent end design. To close in the end of a 30' x 10' tall hoophouse cost me about $239 per end. Here's what I did.
Framing in the doorway
The opening for this hoophouse has to be 10' tall and at least 12' wide to allow my tractor in and out so I can move materials in easily. So in the 30' wide opening, I did two 7' ground plates, two 10' uprights, and a 12' diagonal. In the picture above, I'm describing the framing at the far right and left. Across the top of the opening I put a 16' 2x4. I tied this lightweight frame to the hoophouse with plumbers tape, securing it with screws into the 2x4 framing.
I wrap the roof plastic around the end to the diagonal framing, into a wiggle wire channel that I've attached. I'll use that same channel to secure the plastic across the front of the house.
Next I lay out the door frames. I'll cut them to size, leaving about 4" of space to allow for hinge clearance. I'll also allow for 12" of ground clearance because the ground will rise and fall, and I don't want the doors to catch on the ground at some point in the future. Having made the frames, I check the diagonals and adjust until they're square. 
I attach two corner supports to each door. One at the bottom of the door closest to the other door, one at the top of the door farthest from the other door. These particular supports are made for the inside support of something. I tossed the screws supplied and used 3" long deck screws to secure them. Each corner support cost me $3.
The diagonal support is a 12' or so long 2x4. After I square the frames, I lay the 2x4 on top of it and use a lumber crayon to mark the angles needed for a good fit. I'll cut them with a skill saw.
It's important that the diagonal cut is fairly precise. Take your time and recut it if you're off by more than half an inch. 
After the corner supports are attached, I drilled a 5/16" hole in it and ran a 1/8" cable through it, securing it with a clamp. So one diagonal is a 2x4, the other diagonal is a cable and turnbuckle. 
I put the turnbuckle at the bottom of the door to allow me to adjust it later without having to climb a ladder. 
You can click on any of these pictures for a bigger version with more detail. Andrea is holding the two doors closed. Notice that the cable runs opposite from the 2x4. You use the cable to bring the corner of the door up if it sags in the future. Don't over tighten the turnbuckle or you'll warp your frame.
Here's the door with plastic applied. One end of the plastic is secured in the channel in the diagonal to the right side of this picture. The other end is secured to the edge of the door closest to the other door. This eliminates a seam between the door and the frame, and makes it a little more weather tight. 
Here's the view from the outside. So there's a channel on the edge of the door, one one the diagonal of the door, and one at the top and bottom. You can see the channel attached to the other door to the right. I'll put plastic on it next. Materials per door:
2 2x4x10' $4.50
2 2x4x8' $3.50
2 hinges $12
8 nuts and bolts for hinges, $9
28' channel & wiggle wire, $28
16'x18'x6 mill greenhouse plastic: $32
1 turnbuckle, $6
12' 1/8" uncoated steel cable, $8
2 cable clamps, $.60
2 corner supports, $6
Total per door: $109.6
Framing for doorway:
Plumbers tape: $9
2 2x4x8 $3.5
2 2x4x10 $4.5
1 2x4x16 $3.50
Framing cost: $20.5
Plus a handful of 3" nails and screws.
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