If I want to make a plastic arc profile with a radius larger than a compass or cutter than I have, how would I do that? Application is a truck shelter/box with a roof that has a slight curve to it.

Thanks. Lee

Take a piece of string and tie a pencil to one end and tie a tack on the other end. Adjust it to the length you need and you have created your own compass. By keeping the tack end fixed and swinging the pencil end in an arc w/the string tight, you can mark an arc of any size you want.

Make your own beam compass with 2 pins & a beam or stiff card, or use a french curve.

Use any item that is both stiff and flexible. A yard/meter stick or a long knife blade. Arch it into a gentle arc of the correct diameter and draw your curve using it as a guide rule.

I'm not sure that the giant compass idea would work. It's for scratch building a box to make Italeri's 1/24 Opel Blitz into a radio truck. That would need a very large compass indeed. I did some searching, though. I think an ellipse template could do the trick.

Making ones own compass works.
Take a wooden slat, preferably of hardwood. Somewhere around a half inch wide and a quarter inch thick and long enough.
Drill a small hole close to one end to fit your favourite pen, you can also use the "cartridge" of a ball-point pen. A smaller hole gives slightly better precision so using only the "cartridge" could be better.
Measure the wanted distance and drill a hole to fit a pointy steel pin of some sort. Hardened steel gives a point that will stay sharp longer.

Tape the plasticard to a large enough surface, measure the wanted distance and tape some thick cardboard to the same surface. The cardboard will serve as foot point for the center of the arc, the sharpened pin will get a good grip in the cardboard. If you are using a surface/table where it doesn't matter if there is a pin mark or two then you can skip the cardboard.
Draw the arc. Hold down the slat and pin with one hand and hold the pen/ball-point cartridge with the other hand and move the slat carefully across the plasticard.

Save the slat, pin and pen/cartridge for the next time.
New diameters can be added by making new holes.
Quarter inch increments for the center pin and a series of
even quarter inches + smaller offsets for the pen/cartridge:
1/4 + 1/32
1/4 + 1/16
1/4 + 3/32
1/4 + 1/8
1/4 + 5/32
1/4 + 3/16
1/4 + 7/32
This gives steps of 1/32 for almost the whole length of the stick by shifting positions of the pin and the pen/cartridge.
Smaller diameters of the pin ad pen/cartridge allows closer spacing of the holes. The short distances can be handled with the usual compass
A fancy add-on could be an adjustable slide to hold the pen/cartridge. Choose the closest quarter inch with the pin and adjust the last fractions with the slide.
The length of the stick defines the sizes of the arcs you can draw.
Adding a blade holder to the stick would turn it into a cutting tool.

/ Robin