Where I'm not using some of my cars much, I'm finding that all too often I'm having to jump start everything

Will a battery isolator do much to help the batteries hold their charge? I was thinking of one of these:
Behold!
Mainly because I don't want to hack up my car interiors to fit a big red plastic one. Not to mention re-routing the -ve battery cable!
Opinions greatly appreciated as ever

Cheers,
Josh