actually tbh, having looked at the thread again, a german flag might look good...
would be easier too as you wouldnt need to use black vinyl, just a red stripe and yellow stripe and it would be much easier to apply too as you can put the red stripe on one side and the yellow one the other side, you dont need to be able to reach the middle of the roof. Would take 30mins.
measure roof across, divide by 3 - this is the stripe width
measure length, this is the stripe length
Cut out both stripes and prep
clean roof very thoroughly
spray roof with soapy water, allows you to slide vinyl around before sticking
peel backing off vinyl and spray sticky side of vinyl of with soapy water too. dont be too excessive with water.
lay the vinyl on the roof. best having two people for this one each end.
starting from middle squigee water out and apply vinyl. be careful not to move vinyl.
repeat for other stripe
this is general overview of what is done. The actual applying part wouldnt long at all. The long bit is laying up on the computer and prepping.
You are probs talking about £60-70 if you went to get it done by a sign company, maybe more if you need them to do artwork for it. I can lay it up for you, cut out vinyl and send to you for a good price; thats if you feel confident doing it yourself. Shame your not closer really.
LL

edit - ok didnt notice the flag was wrong lol! still same principles apply.