I was thinking either cutting out the numbers for "15" from fondant and outlining them in royal icing to resemble stained glass, then filling in the open areas with colored piping gels to mimic glass. After it is all dry, paint the royal outlines with gold luster dust mixed with a small amount of vodka.
Alternatively, you could do an entire arched window shape and make "15" part of the stained glass design. I think that is what I would do.
As for making the cake, it will help you immensely if you figure out all of your ingredients and supplies and pick up the non-perishable items now.
You can cover cake boards and assemble some items, like red GP roses, "stained glass", if you will make them. Two weeks before you can bake the cakes and freeze them to help make final prep easier. Let each tier cool completely and then wrap in plastic wrap followed by foil. Label each cake.
Figure out how much fondant, frosting, and filling you will need and spend a whole day making those, I usually do this on Wednesday. Thursday you can fill and ice your cakes. Friday can be spent on decorating. On Saturday, all you have left is delivery and set-up. And figure that it will actually take twice as long to do something as you think it will take.
To give you an idea, It took me Monday-Friday, working 8am to 5pm to make a simple buttercream stacked 7 tier cake, all one flavor except the top tier, and one two tiered Harley-themed grooms cake. And I had the help of a dishwasher, (my aunt.) Monday and Tuesday were spent baking, Wednesday making filling and frostings, Thursday torting and filling and icing, Friday decorating and placing supports. Saturday I went to the ceremony, then high-tailed it back for set-up. (Thank goodness they had a lag between the ceremony and reception, or I wouldn't have been able to go.)
You can totally
do this, but get started! Time will creep up on you before you know it!