I think you guys are overthinking it. All you need really is a more extreme version of what Sam Long already has between the arms.
https://x.com/t100triathlon/status/1779422380181327901
Just move BTA hydration up and back to the same position it would have been had it been down his shirt. If you need to, get a longer/bigger bottle or an adapter that allows you to mount 2x bottles back to back. If you need the extra fluid, fill it up, but there's no reason (especially if you're worried about it structurally) it needs to be filled.
This comes with the caveat that this is obviously much less safe than just stuffing a bottle down your jersey (which is why I think the rule should be changed back), but there's nothing stopping us from just creatively using hydration bottles like Joe Skipper, only between/behind the arms, so long as it vaguely looks like a BTA setup that's already otherwise common.
https://x.com/t100triathlon/status/1779422380181327901
Just move BTA hydration up and back to the same position it would have been had it been down his shirt. If you need to, get a longer/bigger bottle or an adapter that allows you to mount 2x bottles back to back. If you need the extra fluid, fill it up, but there's no reason (especially if you're worried about it structurally) it needs to be filled.
This comes with the caveat that this is obviously much less safe than just stuffing a bottle down your jersey (which is why I think the rule should be changed back), but there's nothing stopping us from just creatively using hydration bottles like Joe Skipper, only between/behind the arms, so long as it vaguely looks like a BTA setup that's already otherwise common.