i guess--if you have one of those coolers that is big enough to sleep in--cake boxes take up tons of room
and the biggest cake box is gonna need to be at least 14" square --that's a very large cooler
http://www.basspro.com/Igloo-Polar-120-Quart-Cooler/product/10229736/?cmCat=CROSSSELL_PRODUCT
this one is 17.4 inches wide on the outside so with all the insulation you might can get a 14" cake box in there--the sides slope inside there though ???
and the thought of them getting wet from the ice is terrifying--i should have said ice packs ;)
and you could place each cake box into a plastic bag and tie it up on top so there's no chance the water could seep in
could it work? sure--but this is how i did it once
i got plastic boxes to set the cake and ice in and then placed all that into corrugated boxes -- it's just getting it all to fit
i got this type box http://www.officemax.com/office-furniture/storage/general-storage-bins-totes/product-prod2350086 at big lots and target
and went crazy finding the right sizes i needed --
then you just slide them in & out--easy peasy and you can use the ice packs and no worry about condensation issues
and you can re-use the plastic boxes for organization & stuff later (15x23x7 is approx what i got)
i made sure the plastic boxes fit into the moving boxes (went to a storage place to get these) that would fit into my car
i used nubbly shelf liner underneath the cake boards --like you set glassware on to keep the cakes from sliding inside
you can fit top & bottom tier in one box then the other two will fit in another one--put ice in another smaller box--seal it all up in the cardboard box--8 - 12 hours or perhaps more even should be no problem if the cake is fully chilled in the first place
*ice packs wrapped in a paper towel to absorb condensation then placed in a plastic bag to control any further moisture issues
i would keep the gum paste flowers separate from the cake in it's own room temperature cake box--the flowers all wrapped up in paper towels--if you want even more security--make 'cups' out of aluminum foil to set the wrapped flowers in--or use scrunched up foil to make channels for the paper towel wrapped flowers to sit within in the box--like depending on how many--i see them sitting nestled in to a nice big half sheet box or quarter sheet box
so all that to say --climate control is huge for you
assembling the cake is kind of a big deal on your wedding day but i know you can do this but another idea is to get a trusted friend/family member who could be your back up--who could be there specifically for any cake issues--already trained--so if it came to that you have a plan