You could try putting the tray outside first, get her used to it, even put some of her crap in it (not nice I know) then when you have to keep her in after speying she might be better at using it as it smells of her.
I had a similar problem when I had the wolf snipped, I was told to keep him in for a night and he just wouldnt have it, was scratching at the door. In the end I had to escort him out, stand suspiciously next to the garden over the road whilst he did his business at 10:30pm, then grab him and take him back in immediately afterwards, worked remarkably well - if she's very clingy with you, you might be able to do this, although I know neutering is a bit more complicated for lady cats and if you have to keep them in a bit longer.
You could try putting the tray outside first, get her used to it, even put some of her crap in it (not nice I know) then when you have to keep her in after speying she might be better at using it as it smells of her.
I had a similar problem when I had the wolf snipped, I was told to keep him in for a night and he just wouldnt have it, was scratching at the door. In the end I had to escort him out, stand suspiciously next to the garden over the road whilst he did his business at 10:30pm, then grab him and take him back in immediately afterwards, worked remarkably well - if she's very clingy with you, you might be able to do this, although I know neutering is a bit more complicated for lady cats and if you have to keep them in a bit longer.