Blake has a nifty tool that is useful for installing the cowling. Rent or buy it here.
The tool mounts to the face of the crankshaft/flywheel on commonly used Lycoming and Continental engines. It has an adjustable disc that simulates the spinner position. The cowling can be clamped, or clecoed, to the disc while you trim, fit, drill it to the airframe, or other cowl half. This allows the cowling to be positioned very accurately and solidly while working on it.
You could even install the cowling before you buy your prop. Of course, you need to know the dimension from the flywheel face to the back of whatever spinner you plan to use. Planning and common sense required!
Anyway, it's a very handy tool that eliminates the need for you to cobble together a homemade tool, or actually work around an expensive prop/spinner.
Obviously, you need to have the engine hung to use this tool! Doh... I hate to have to point that out, but someone will ask!
YMMV, but many of you may find a tool like this very useful and time-saving.
F1 Rocket and F4 Raider components
RV and Rocket Accessories, Tailwheels, Tools, & More
F4 Raider - under construction
F1-H Rocket "Crazy Horse" - sold
RV-4 "Chief Pontiac" - sold