Good greenhouse idea (probably) but need advice

I've made cattle panel greenhouses in the past, but they've been snatched up before being used for their intended purpose. (In fact, if you have especially keen eyes, you can see one in the background being used as a chicken run for our new rooster).

This year I not only put them over plants, but I thought I could function-stack that with straw bale gardening.  That way the bales add support for the hoops and, come the (now approaching) winter, straw bale decomposition adds extra heat to the greenhouse.  Win-win!

They have worked well all summer, and now it's time to add the greenhouse plastic.

From the close-ups you can see:
1) I've added some plastic wire loom tubing to prevent the plastic sheet from finding any snags on the ends of the cattle panel
2) I've drilled and tied up some grey PVC pipe hanging from the peaks. The intention of that is making Eliot Coleman style scissor doors (official-ish link).
So I vaguely see some potential going there...

Now for the problems I need help with:

First: how in the world do people hold greenhouse plastic onto cattle panel greenhouses!??  I tried folding it over and fusing it with an iron, but there's a razor-thin margin between too cold to get a good bond and so hot that it melts holes.  I think I'm very much on the wrong track here!  What's the best way to secure the plastic?

Second:  I foolishly placed them too close together.  During the summer, plants reaching out made the alley between impassible, but not a show-stopper.  Now I'm a little worried about snow filling up the spaces in between and smashing things.  I've added a couple of sketches how I think a person could cover all three with a common ridge pole and a few weights to cause snow to funnel down between.  Not sure how I feel about that idea.  So also, what opinions and/or advice do you have for that?

Addressing what I have done with a single high tunnel, I have placed wood walls with windows and doors at both ends.  Along the sides at the bottom I have wood going up about 1 foot. This provides protection for the plastic from string trimmers.   I stretch the plastic over the top and fasten to the wood at the bottom using grommets.

Duck tape from inside.  Use short strips placed perpendicular to the wire leaving 1/2" gap at center where wire goes through.  Illustration:

         |||             < duct tape, tight
---------] ] ]---------  < wire with loose tape
         |||             < duct tape, tight

Experiential warnings - The cosmos has spoken!

1) Do not attempt to undo any mistakes.  Move next to it and do another.   No takesy backsies.
2) Less is more.  Too close together will create too much tension.  Ever seen the flying nun?  Listen to the cosmos!  You will have to experiment.  
   I don't know your wire gauge or plastic mm.  Start with maybe every 3 - 4 ft??? laterally.  Evenly space 4 on the rise (2 on each side) depending on how high.  You can always add.
Caveats:  Max common wind velocity.  Wire gauge.  Plastic thickness.  Never tried covering 3 at a time.

I have also had good luck using the stick-on cable tie holders that you can get on that union busting online store that delivers in 1? 2? 3? never? days.  Be sure to get only those that have strong adhesive, preferably superglue.  Should be around $10 per 100.
More from the cosmos:  Wear cheap disposable medical gloves while attaching.  It is easier to remove and toss glove with holder stuck to it than to get holder and any remaining adhesive off your finger.
Be well.  May your cosmos be kinder to you.

Hi Marla and welcome to Permies! It's nice to get an answer from someone with well earned experience!

Here's mine based on a conventional hightunnel:

Wind and snow will not be your greenhouse's friends!
I suspect in your area particularly, snow loading may be a problem, and possibly also low temperatures which will embrittle the plastic sheeting. Is it worth considering whether you need to cover all three for the winter? If you only cover one, or just the outer two, the snow will naturally shed more off them.
My tunnel doesn't attach the plastic to the tunnel at all. it is buried around the edges  - I think this makes it more secure in high winds which may not be such an issue with you, but 80 mph winds are not unusual here over winter. Around the doors the plastic is wrapped around a wooden batten which is then nailed to the frame. This gives it continuous retention so spreading the load over the whole length. Point loading and creases are likely to cause issues.

I wonder how the people on the link given attached their plastic to the end frame? Mind you, that is full size hoops as well isn't it? I think they use a snap on (or snap in) wire these days as well. maybe some edge protectors like car door seals may work to attach at the end hoops?

Hi Marla,

Welcome to Permies.