Interesting project Graeme, I had a completely different picture in my head while I was reading through the thread and commenting. I was under the impression you were building a new windscreen from the fairing up. I can't say I love the way it looks, but if it's functional who really cares it's a prototype model anyways
With enough practice and experimentation you'll have a compound curved, full on custom screen on there in no time.
I just had a thought that may help with future models. If it were me I would cut the perspex to the desired shape before bending it. My reasoning for this is that it is much easier to achieve consistent edging on flat material than on curved material. Also if you do all the cutting and edging before heating you can leave the protective paper or plastic on the material and minimize scratching. At work I use a 1" wide belt sander to clean up rough cut edges on pretty much all types of glass and perspex. Like shaping the material, it takes practice to get a consistent edge. The easiest edge shape to make is a "flat grind" it's just a 1-2mm 45 degree chamfer on both sides of the material and a flat spot between them. A "pencil grind, or polish" is an evenly rounded edge that IMO looks better than a flat grind but takes much more practice to make look right.
As far as the how to goes, I would use a clamp or two to secure the material to my workbench with as much of the edge hanging off the table as possible. Using a 120 grit belt start by making the edge nice and flat, then finish with the 45 degree chamfers. There will be some melted material that sticks to the edge. To remove it simply take a fresh flat razor, hold it perpendicular on the edge and drag it along the edge. Always keep the sander moving back and forth so you can keep an eye on your progress, also use light pressure and a slow speed setting (if your belt sander has a variable speed setting). A conventional 3" wide belt sander will work if you don't have a 1", but it's a little bit clumsy, and hard to use moderate pressure with. The best tool for this job is an upright belt sander that lubricates with water, the one I have at work is a Bee metal sander most people don't have one kicking around in their backyard shop though. Another tip for using the 3" sander is to use a finer belt, it will plug up quicker but it won't chew through the material as quickly. If you want when your satisfied with the shape of the edge yo can make it glossy by heating it lightly with a small butane soldering torch. When doing this you'll want to keep the heat moving pretty quickly to avoid bubbling the material. Also the heating in the oven stage may have the same effect on the edges so this last little step may not be necessary.
