Congratz on your upcoming engagement!
One of the most beautiful Chuppahs I have ever seen was done here in NYC for a wedding at a hotel. The floral design used these fake trees in the ballroom, covered them with flowers and lovely hanging fairy lights ( the hanging glass votives) and a simple cloth strung between the trees. It was stunning.
You might be able to use 2 rose trellis or arches from a garden supply store and drape with fabric?? A simple muslin will look light and airy. you could string christmas lights under it for lighting ( specially if in evening....) Or if inside you could buy and inexspensive canopy like for a bed (http://www.dhgate.com/free-ship-4-post-bed-mosquito-net-four-corner/p-ff808081243d4f4e012446eb2e931dfd.html
) and doctor it up to match your theme / colors.