Coroplast over a steel tube frame seems to be the 'go to' method of building Boat Tails etc here.
There's a lighter, easier method:
Light Aircraft and gliders/sailplanes use a similar underlying frame, but this is then covered with an aircraft fabric that is pulled so tight that it does not crease or flap in the wind.
It's Pulled that tight first by ironing the fabric which shrinks it, then by 'doping' the fabric which pulls it even tighter and makes it airtight.
Its then spray painted much as one would normally paint a car.
This would make for a lighter, neater, more professional looking, boat tail etc.
No steps and bolt heads etc means more aero too.
ie: You build your frame out of tubing of some sort:
steel, aluminum, glass/carbon fiber, balsa wood, recycled fishing rods, bamboo or whatever.
Then stretch aircraft fabric over it using this tried and tested method.
Here are a couple of links on the process to get you started:
https://www.youtube.com/playlist?lis...8E69E46A3F6FC2
https://www.google.com/search?client...&bih=906&dpr=1
With my background in building gliders etc I don't know why I didn't think to mention it sooner!