I'll be taking delivery in a few days of a handheld BF-F8HP for GMRS communication on this island and maybe goofing around when skip conditions are right. So, question: Is there a 6 or 7-foot omnidirectional antenna matched to my handheld that I can attach to the wooden railing right outside my door and then can take down at the end of the day?
You can try the antennas from Ed Fong. He sells several models from base antennas to "roll-up" types for portable use. None require a ground plane so they are ideal for temporary use.
For a base type antenna look at this one.
"DBJ-1 (commercial) dual band base antenna - (152-157 MHz and 460-470 MHz). Maximum power input 75 watts. - $36"
For portable use< roll-up type, this should work.
DBJ-2 (commercial) dual band roll up antenna kit - (152-157 MHz and 460-470 MHz). Includes adapters for BNC, SMA and SMA - female. Also includes a 6 ft extension cable. Maximum power input 50 watts. - $36
The link to the website is below.
These are "J-Pole" antennas. For some background on what they are a basic theory how they work can be found at the following link.
Without seeing your situation a roll-up antenna might be a good choice if you have something nearby to throw a light string or rope over, like a tree limb. Then you just hoist the antenna up by the string, the higher the better. When you're done just drop it down and store it. Likely you can get away with leaving the string in place for next time.
Some years back when I lived in a ground floor apartment I used a portable light fiberglass "pushup mast" stuck in a heavy duty speaker tripod. That got a telescoping dual band antenna up about 20 feet. I ran the coax out of the window. This was done at night so as not to attract too much attention.