my mate sophie did.
she bought some fleece from Boyes (its like a biiig craft oriented shop) and a surcingle and used a buckle from an old turnout rug that had a hole in.
laid a cooler she already had over the fleece, cut round it leaving 3cm extra for hemming. hemmed it on her mums sewing machine, sewed on the buckle by putting it in roughly the same place as it was on her bought cooler and stitching some plaited baler twine on as a filet string.
popped it on Briggy her horse and fastened the surcingle round his middle. cost about £10.