Very nice Gary........looks like something I would have bought if they had been available before I got me risers .
I'd agree about the stress point on that inside angle though...that'll be where it'll go if anywhere I reckon.
If you made them curved instead of having the 90 degree angle, it'd vastly decrease the chances of any cracks developing there, and would look better too (but that bit's just my opinion obviously

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Quite seriously though, do look at some sort of insurance.......I'm sure your man is capable and the materials good, but you have to think about the lowest part of the equation, and that'll be some idiot rider fitting them wrong, overtightening or undertightening them or something......
Insurance shouldn't be that expensive, but if something did go wrong you could truly lose everything if you didn't have any.