Zoë, I see you have been going with many items smaller than your actual size. Perhaps, though surprising, this is the actual problem? I may have misunderstood, but
well, maybe it would actually work if it were the right size?
I just thought of a crappy plan. It may be useful but probably isn't. What about wearing tight boxers, tucking in it, holding it like that somehow and then putting on two
pairs of thongs (inverted, the "string" forward) to secure it on both sides?
(This is probably a ridiculous plan)