There are other sealants that are similar to ProFlex...OSI Quad is one that (at least from reading the tubes) is similar and is readily available at big box stores for less money than ProFlex.
I've heard of (and used, with success thus far) plumber's putty for these sorts of small exterior bits and pieces. It's readily available, easy to work with, and easy to rework if needed. You don't need huge gobs of any sealant.
Avoid silicone. It's not that it is particularly bad at sealing, so much as nothing (including more silicone) will stick to it, and eventually you will need to replace/touch up any caulking. If you use silicone, you'll then be in for a very long and annoying cleaning/scraping process to get it all off.