Sloped Roof for Outdoor Freezer - Durolast or what else?

I own a building with outdoor large walk in freezer (16' x 24') - condenser is (now) on the ground.  The roof is holding water and seal between panels has leaked twice.  It is just 3 years old.  There was a rubber membrane recommended for box by manufacturer but the terrible installer we hired declined it and now I have an issue to solve because my roof has some asphalt on it and water is penetrating between the seals.  It needs to slope.  One of the 16' ends of cooler butts against a metal building because it has a freezer door that opens to interior space.  I don't want to break the bank.  The installer recommended a durolast roof and I am getting a quote but any other suggestions from you pros?  I can't deal with the installer he is not allowed back too many issues. Just trying to solve the problem best way that isn't crazy expensive and will last and keep the water out.