I can play a harmonica but just couldn't work a melodeon.
Whistle/tabor pipes etc don't have much volume if you're in a crowd.
Why not try a small piano accordion? You can just play the tune on the keyboard side to start with and it's louder than things like mandolins and bouzoukis.
For portability, look for a student model or a 24 bass (or 48 bass if you're a big strong fella) but don't go for a gent's 120 bass. That's overkill and Morris does't require that many chords.
Different brands/models have different sounds, some gentle, some that have an Italian sound, others that have a sound that you might hear from Jimmy Shand.
The one I use for morris is real grunty, with a strong bass and you can squeeze it for phrasing so it sounds rather like a melodeon. It's a 24 bass Mazzini, with only 2 octaves on the keyboard side but it plays most morris tunes. No, you can't have mine! It actually belongs to our Squire and he won't even let me buy it from him, so it's just on loan. My own one, a 48 bass Hohner Student VM is just too heavy for me to stand around holding for very long and anyway, it doesn't have as much grunt - I can't get a sudden squeeze with a bit of ooommph the way I like, for tunes like Saturday Night On. I use that one for diddley sessions where we're all sitting down (and using a few more keys).
Both boxes cost less than $100NZ.
Robyn