Very difficult is relative and given enough time players of most strength should be able to find a mate in one.
I don't have a concrete example, but in my view the following features would make it more difficult (some of these exclude each other):
a large number of possible moves (i.e. lots of pieces on the board and an open position)
the mating move is ...
Three moves is probably in the realm where you could manually brute-force it, although it may take some effort.
Pick a move, a reply, a second move, and a second reply. Then start looking at whether White has a checkmate from that position. If not, then White's second move was incorrect, so change it. If you eliminate all possible White second moves this ...
The only way to prove it with 100% accuracy is to use a special engine. Since you do not want to see the solution yourself, then you could ask someone you trust to use it for you. Having a friend sit there and attempt to solve it will not really prove anything since he is human, and may not be strong enough to guarantee a correct answer.
You are almost ...
I did some further searching and found an article about an endgame tablebase that takes into account the 50-move rule that was created by Galen Huntington, which is an interesting concept. The article gives a position that is mate in 154 that accounts for the 50-move rule-a new length record!
However, I am not taking into account the other positions that ...
Here's an easy mate in 20 with a unique solution
(which becomes a very easy mate in 19 after the first move):
[Title "Noam D. Elkies, Chess Stack Exchange 2020"]
[FEN "6bn/p4p1k/p4Ppp/p7/p3K1P1/P4N2/1P6/7n w - - 0 1"]
Looking at the descriptions of patterns from wikipedia, this exactly fits "Mayet's mate". The section on Andersen's mate suggests opinion is divided as to whether Mayet's is a form of Andersen's or a distinct pattern.
Because he is losing the Rh8, and at that level, that is an EASY win. It is also worse than that as mate is coming in a few moves. Even without the almost immediate mate, being down a pawn and an exchange is a lot of material.
By the way, although black did not take it, white did offer a queen sac on move 24.Qg7, but had it been taken, a new queen would ...