The volvo came with an injection elbow installed. You can see it in the picture. I think fabricating an elbow is a waste of time. One from Hamilton is perfectly adequate. I have a Westerbeke elbow on Seaquestor at the top of a 15" black iron riser.

Depending on how the riser is connected to the exhaust manifold, you may need to support the riser in some way. On Seaquestor the riser is attached to the manifold with a clamp and gasket. That allows vibration to cause the riser to move (rotate). To prevent that I made a loop pf 5/32" stainless rod that goes under the injection elbow and then up to the bottom of the bridge deck where it it attached with a shock absorbing attachment (to minimize vibration transmission = noise).