If you find those you like, drape the plant top flower stem with a very light fabric to keep the bees out...remove when the flowers open, then dry. Any seeds on that inflorescence will be pure

i have seen plastic bags over flower bunches, so was figuring there might be something like that out there. light cloth it is.

i want to keep both my fave cukes, but i can space them pretty far apart w/o a lot of trouble.

no good would cook the flower.. need air

For cukes, you can pollinate them and mark the fruit as pure if the pollination succeeds. Many cukes are hybrids, so I hope your favorites are open pollinated.

You can cut from the basil you like and grow a new plant. Keep them indoors over the winter and plant them again in the spring.