I have built 3 of the cattle panel greenhouses over the past few years.
I do not have as much wind as you, so I haven't needed to use a center purlin reinforcement. I just bent over the cattle panels and secured to the wood baseboards. For some reinforcement, I overlapped each cattle panel about 3 inches and secured with zip ties.
To cole_robbies point, secure the plastic as tight as possible. Also, make sure the smoother side of the panels are facing outward (for less damage to the plastic).
I have found the limited height to be a challenge. With the apex at 5 foot 8 inches, I have gashed my head a few times when walking through. I ended up raising my entire tunnel a few inches by adding a second set of wood base supports (the railroad ties in your case) on top of the first set. I made the top row a little narrower than the bottom row so that the bottom of the panels would sit on top of the bottom row and push into the side of the top row. (A picture would serve better, I'll see if I can find one.)
The other question about height is that if 5'8 is the apex, then your plants cannot get very tall before they 'hit the ceiling'. Not sure what your plans are for growing. I use my houses for growing shorter varieties and dwarfs.
I combat the heat by using shade cloth and/or running cheap fans on a timer.
Good luck.
Bill
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