The boot fit on the porcelain alone should be enough to hold the boot/cap on the spark plug.
Some of mine are so tight I have to use a boot puller tool to get the plug wire off the plugs.
I used spark plugs with the screw on tip for years without any boots coming off. They also make a plug boot/cap that uses the straight bolt end without the threaded tip installed. These use wire cap inserts that slide down over the plug tip threads. I think most Japanese motorcycles use them.
If you spark plugs have the treaded tips, find the boot/cap for just the threaded plug without the tip screwed on.
