Cheapest option is to buy a pair of risers off ebay and either bolt them on, if the yoke allows it, or get them welded on by a local engineering firm if you have one. The nicest option is a pair of billet yokes of course. Andy's (Spondon440) are on my latest build, and quite lovely If you want another option, ask Garry at Metmachex. He can do parts like that, but one offs will cost more than a batch, so any firm doing a one off machining job will be your most expensive way of doing it. Batches of 10 or more are often the most cost effective way of doing it.
I'm looking to get bar risers added to a TZR125 top yoke, but may consider a custom (billet?) one if it's a viable and a cost effective option.
Any experience and typical costs for getting risers added (welded) on or custom top yokes made up?
The problem with fitting risers to a 4FL/HW top yoke is that it is not flat which makes fixing an issue. Also the top yoke has an offset ignition switch mount & nowhere to fix your clocks. I'd contact Andy, spondon440, he made the one I used & it fitted great.