What model L-5? Ambulance or observer? You can put it externally on the underside of the aircraft attached to the recognition light panel. If you have an ambulance, especially a late model (E-1 or G), you could drill a +/- .75" dia hole in one of the aft greenhouse panes and put it there and make it easily removable (quick disconnect) so at shows you could stash it under the litter deck and say the hole is a field mod for the BC-659 Command Set's Telescoping antenna. What I did, is put the antenna inside the vertical stabilizer attached to the wooden rib at the base. It works pretty well with my radios, but not the best. Unless you're restoring the bird to be completely authentic, there's no reason to sacrifice functionality for appearance in my opinion. Good luck and if you have any more questions let me know
I had my antenna coax cables and ELT cables cloth covered and hid them at a bottom of a wire bundle that runs (on a G model) from the original radio modulator (located between the pilots legs) to the aft litter deck and to the tail. It works quite well, but I may put a removable external antenna on my aft greenhouse pane in my BC-659 antenna hole (as mentioned earlier). I'm having a bit of trouble receiving with the antenna in the vertical stabilizer. Sometimes I have to turn 90 degrees to get a signal, but if it were easy everyone would do it right 
