Now that is a difficult hand, as Double is not known by GIB.
I considered bidding 4N, and the explanation of that is: Unusual notrump, 5+♣, 5+♦, 4+♥, 18+ total points, forcing.
Well I not only considered it, I bid 4N, and North bid 5♣, not showing ♣, but a cuebid, but not mentioning what was trump.
Testing my options now I can see that 5♥ also would be a cuebid, so probably it is ♦, that is trump.
So I decided to signoff in 5♦, and what happened now?
Well North bid naturally 7♦, just one down, perhaps I should have tried to sign off with 6♦