Get ahold of you tailor and inform him you will be having a rental tux for a wedding and you will receive it two days before. Now, with this you take him the rented tux, fork out the $$ to get that rental altered to you. Don't say a peep to anyone, it will be the best you can do. If you don't talk, MW will never know!
Just make sure when you go to try it on at MW, you get the closest to best fitting one, to minimize alterations. Bringing in the waist, taper the arms and length of sleeves along with a trouser taper, will really make you stand out amongst the rest of the Potato Sack's.
-this is what I would do if I were in your shoes.