An idea open for discussion and/or ridicule.
Get one of these umbrellas:-
A brief guestimate of sizes would suggest these brollys are about 30" wide. Unfortunately that makes the the wrong size to come inboard of the front wheels so you'd need to make a lightweight frame to make the edge taller and narrower effectively squashing the umbrella in at the sides with the cut-out towards the road. Chop the handle off as much as possible, then from the frame you've made attach to the main boom top and bottom placing the brolly in position as a nose fairing and allowing pedalling. Now make a frame around the rear of the seat, sloped to match the seat back. This needs to be wide enough to clear your shoulders and any steering gear (direct to steering tube trikes are going to be a problem here) and tall enough so your head pokes above it. All you really need now is some similar waterproof nylon material to velcro to the inside front and outside rear edges of the frames with a cut out for your head to poke through. You'd only go as low as the "full" part of the umbrella. This material is a perfectly flat with no compound curves to be seen anywhere as they're all in the umbrella. That material would need fibreglass ribs sewing or glueing in to match the umbrella spokes to add rigidity. The sheet required is flat but an odd shape. It's not too hard to make a paper template first though. You could taper the rear further from the seat if you should wish with a further effort but make the access fastening as close to the seat as possible so it can be reached. The only issues I can so far foresee are getting in and out would require the rear to be undone at one side so I'm thinking snap magnetic strips on the rear at the chosen side not velcro, it's going to be tight to miss USS but clear the front wheels as per any such design and the brolly frame is going to be hard to make. It needs to be strong enough to hold the brolly in it's new shape without destroying the strength of it. The umbrella, material and poles will come in at around 2 to 2.5 kg so if you can keep the weight of your frames down it's not a chore to move it. The flat material part can even be stowed on a sunny day somewhere behind the seat if the ribs placed in it are collapsible. The brollies also come in clear should you wish and you could make the flat part out of clear too. The umbrella joints could be reinforced with epoxy or a bit of JB Steelstick or similar.
Alternately you could also mount one of the same umbrellas at the rear and then the flat material would be rectangular. Getting in and out would require a solution as the rear joint is too far away to reach.
Over to others.
Get one of these umbrellas:-
A brief guestimate of sizes would suggest these brollys are about 30" wide. Unfortunately that makes the the wrong size to come inboard of the front wheels so you'd need to make a lightweight frame to make the edge taller and narrower effectively squashing the umbrella in at the sides with the cut-out towards the road. Chop the handle off as much as possible, then from the frame you've made attach to the main boom top and bottom placing the brolly in position as a nose fairing and allowing pedalling. Now make a frame around the rear of the seat, sloped to match the seat back. This needs to be wide enough to clear your shoulders and any steering gear (direct to steering tube trikes are going to be a problem here) and tall enough so your head pokes above it. All you really need now is some similar waterproof nylon material to velcro to the inside front and outside rear edges of the frames with a cut out for your head to poke through. You'd only go as low as the "full" part of the umbrella. This material is a perfectly flat with no compound curves to be seen anywhere as they're all in the umbrella. That material would need fibreglass ribs sewing or glueing in to match the umbrella spokes to add rigidity. The sheet required is flat but an odd shape. It's not too hard to make a paper template first though. You could taper the rear further from the seat if you should wish with a further effort but make the access fastening as close to the seat as possible so it can be reached. The only issues I can so far foresee are getting in and out would require the rear to be undone at one side so I'm thinking snap magnetic strips on the rear at the chosen side not velcro, it's going to be tight to miss USS but clear the front wheels as per any such design and the brolly frame is going to be hard to make. It needs to be strong enough to hold the brolly in it's new shape without destroying the strength of it. The umbrella, material and poles will come in at around 2 to 2.5 kg so if you can keep the weight of your frames down it's not a chore to move it. The flat material part can even be stowed on a sunny day somewhere behind the seat if the ribs placed in it are collapsible. The brollies also come in clear should you wish and you could make the flat part out of clear too. The umbrella joints could be reinforced with epoxy or a bit of JB Steelstick or similar.
Alternately you could also mount one of the same umbrellas at the rear and then the flat material would be rectangular. Getting in and out would require a solution as the rear joint is too far away to reach.
Over to others.