Ziggy, I had a play with the XT and LX cranks last night... the splines are the same but the depths are different which means that although the LX spider fits the lockring is not able to hold it on - I have used a cassette spacer before as a packer but it means the C-ring won't fit (Cassette spacer needs a little modification also).
End of story is that this is not the ideal solution although it could be made to work.
It was my plan to do this for my commute bike when the chainrings on it die - which will be soon as the whole drivetrain is running out of teeth - but I have bought some more chainrings so this is now surplus.
Crank Spline Lockring and C-ring
I only have the drive side for the LX cranks - teeth are excellent - but there is no C-ring for the splines - you can have these cheap as I now have no need for them.
The XT cranks are completely worn out with almost no teeth left.
But they look fine with the 36 tooth Forge ring
He's not the messiah, he's a very naughty boy.
95 Kona Explosif Singlespeed/Fixie - 100% Thrashed since new and it's still very well loved (and it looks it).
Dave Lloyd C.A.T.S W.I.S.K.A.S - Built, ridden, loved, and sold.