Hello Ac,
Is the polytunnel project for your friend going well?
We moved to a property in the North of Scotland that came with a large polytunnel. 3.5 meter by 9 meter? Something like that.
I tried to make it work for 5 years without great pleasure or succes. 2 months or so I cut the top off and so far I'm much happier with my nunnel (non-poly tunnel) than I ever was with the tunnel. My cabbages have no mildew for the first autumn ever.
Now most people seem very happy with their tunnels, so there must be something in it. For me it wasn't worth the effort of watering, airing and the continually trapped insects and animals.
The inspiration to cut it came from the Shetlandish plantie crubs, that are basically surrounding walls to keep the worst of the wind off. You can put over netting if you were so inclined. (I'll try to ad pics)
For me, some conjunction of plastic tension and gravity have allowed me to keep a poly sheet up on the wind side for now, the other side is garden netting I had. In spring I'll add another layer and grow peas and runner beans up the side.
Probably not in the same order:
For airing/positioning I would try to catch the prevailing wind on the head side so you can install 2 doors and have them open so the wind can blow through most of the time. Netting bottom sections seem to be a trend too, I'd definitely want those in a tunnel.
Watering is a major, major job. And that's when everything is working. Repairs and unforseen circumstances must be exponentially harder when disabled, so this would be a major concern for me. A passive watering system with no moving parts sounds a solution, so a seep bed would definitely be high on the list.
Gravity and tunnel placement could work really well on this if you can get a flow to/through the tunnel somehow.
Animals get trapped, even with two doors open at all times. Birds and rodents can usually figure it out, but insects just try to keep flying to the light. People tell me they'll get out at night if i don't catch-and-release, but even then I can't see how a day locked in a hot dry tunnel is good..
Opening top panels like in a geodesic dome may be a solution. Or no top
Poly tunnels are relatively expensive and the plastic will need replacing regularly. More cost and such a pain to do, even with 4 non-disabled adults. And don't forget your plastic will need cleaning. Our big tunnel was too big and even with a ladder my very tall partner couldn't reach the top outside to get it properly cleaned. And just a bit of grime throws a lot of shade
Raised cold frames may do a similar job for a fraction of the price and upkeep. If placed well and raised I can imagine these as very wheelchair friendly.
Planning your crops to suit the circumstances really helps. I got Covid at the height of the growing season a few years back and was out of gardening for nearly two months. What a mess my annual beds were. Now I grow perennials whenever I can.
We grow portuguese longer-lived kale/cabbage. I'm not sure how long they live, but even if not fully perennial they'll last a lot longer than annuals. The leaves you want to pick will grow higher, so no bending required. They didn't like the tunnel, but outside they thrive. So less planting, easy harvesting.
Our garden is shady, but my neighbour is getting elderly so he offered me use of his sunny glass house. Smallish, but walk in. So I grew 8 tomato plants in there. And sure I could have watered a bit more and gotten a better harvest, but even for a potentially double harvest of what I got, it was A LOT of watering.
Will I grow tomatoes again? Yes, because delicious. But I'll find a way to make them self-sufficient, or at least self watering.
Will I tell people to also grow tomatoes? Not without warning that it is a lot of work if you do it under cover
So, if I were you I'd start by building your friend a few big raised cold frames and a few raised beds near his house. And get a passive water source going.
I hope you do well and do send us pictures when finished!