I've experimented with many solutions and the most effective was still a little inelegant. I have a canvass pouch designed to hang from a belt or bag whilst out hiking. This pouch holds a water bottle. So when I cycle to my rehearsal with my tiny travel bass I take the bottle from the bike, fill it with water at the rehearsal room, clip the pouch to my guitar strap (at the bum end of course) and the weight of the water precisely balances the instrument without me having to carry anything heavy on the bike (which would kind of defeat the object of travelling light). I carry the bottle anyway and it becomes lighter throughout the ride as I consume the initial quantity of H2O.
So I rehearse with a bottle swinging at my hip. Perfect if, as I say, inelegant. I am therefore adapting the idea for when I'm actually gigging with a neck heavy bass. Quite simply velcro attached to the back end of the strap and a certain number of curtain weights sewn into a flat pouch with the other half of the velcro sewn to the pouch. Balanced bass? Remove the pouch, neck heavy? Velcro it on. The 'certain number' of weights is arrived at through trial and error but by using three smaller pouches rather than one big one I can stick on differing amounts of weight to suit each bass on a case by case basis. Or a bass by bass casis.
A bag of curtain weights, yesterday:
Pictures to follow. When I've actually done the sewing.