Hi Drew- fantastic that you are going to create this piece yourself.
I did it and it is indeed a special feeling to get the chance to do
this. I agree with tips outlined by Marta and will add my piece in
good faith.
Don’t let alloy problems curb your design initiatives. These
problems are always taking different shapes and forms and need to be
addressed when they present themselves. A tension setting is nice
idea but can pose problems and risk to your expensive stone. You can
create a “false” tension setting where there is a base or small
pedestal where the point of the diamond is seated and the stone in
set in a ‘half’-bezel setting. See scetch. Another option is simply 4
claws or prongs. Classic- yet dazzling setting when combined with a
contemporary design of shank. Just try keep it simple. Simple is
your friend. Overdone may look tacky (this is opinion). Depending on
the colour of your diamond, stick to white gold in metal choice. A
good tip that is often overlooked is to polish the inside of the
setting as best as is possible. This adds to the ‘life’ of the stone
and may blind your fiance 
Another tip which may sound stupid to a
lot of readers- check the stone. Make VERY sure that there is
absolutely no play. Tap it lightl with you finger next to your ear
and listen to make sure there is no ‘tick tick tick’. Also, run
tweezers over the table of the stone and observe under
magnification. NO MOVEMENT. Whatever setting you choose, make sure
that the claws/bezel are abslotuely flush with the stone. Then, with
a sharp graver which is polished on the belly (underside) for bright
cutting, cut around the stone on the inside of the bezel so as to
rid your work of any unwanted burs, and to give in a uniform and
bright finish. See scetch.
Lastly- if you have never done this before- try and get someone to
show you on another peice etc. If you have time, practice with a
cubic or similar cheap stone. Remember, the crappy scetches above
are a simple design used for example purposes only- I’m not
proposing that design anything remotely like this- it’s a design
that’s been done a million times- that said it can be quite an smart
ring.
Good Luck with everything.
Paul