I'll preface this by saying I don't know much about plucking, other than what I have read, and I do not know your bird's history, but instead of a temp fix, why not try to find a cure? I'd imagine a "sweater" would be horribly annoying to a Budgie and wouldn't stop the problem long term. If you prefer a temp fix, have you tried the feather sprays (which I must admit,
I have no idea if they are safe or what is in them, so I'd check that out first)?
Since plucking stems either from disease (pain, illness, malnutrition, infection, Psittacosis, allergies, hormones) or behavioral issues (boredom, loneliness, cage location, stress, light) I would take them to an Avian Vet, if you haven't already, as a first step. Have you tried moving their cage, putting them with other birds or separating them if they are already together, changing their diet, more or less light, playing classical music, etc??
I know Bird Talk magazine had an article about plucking several months ago or so and I believe it said sometimes you just have to let them be and love them despite the plucking, given they are not hurting themselves.
she is close to 1/3 featherless now (her legs look like mini drumsticks! i know it's not nice... but it does look kinda silly!)
Aw, poor girl! She's probably adorable regardless of missing some feathers. I hope you find a solution that works!