The good news is that there are a number of hotels around the Platinum Mall in Bangkok and the choice extends from fairly cheap all the way up to 5 star hotels.
The absolute closest is The Grand Diamond which is right next to it, but The Amari Watergate is right opposite. It's possible to walk across the road, but if that is not to your liking, then there is a footbridge leading directly into Platinum Fashion Mall just a few yards down the road from the hotel.
Hotels Nearby Platinum Fashion Mall
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AMARI WATERGATE HOTEL
The Amari Watergate Hotel is directly opposite the Platinum Fashion Mall. Just across the road.