I made up some simple bar risers by using a stack of thick stainless washers, and replaced the lower clamp bolts with some 75mm/3 inch M10 bolts with a 14mm head (I had to grind some 16mm ones down as no one had any with a small head like that).
Simple to do and a lot cheaper than buying risers. Ride position is more comfortable and the bars are higher by around 1.5cm, which makes a lot of difference. There's no additional flex and the washers are not noticeable. I may see if I can get some alloy collars machined at some stage.