Building on Gordon's good plan of the wire, I would substitute a flat gauge instead of the wire that would mimic the inside intersect of the two angles (looking at the right inside of the drawing). This gauge would be inserted into the part and held in place with a wood wedge (or metal) entered from the opposite of the being measured end. Then with using a plug gauge For+- .005 or a depth micrometer for +- .001 an accurate measure could be made.
Yes, splitting one part in half would be an aid to make this gauge.
Likely I would make this gauge a little long and then feel in the size to shoot for a simple for the convenience of the operator (like hit a zero). The gauge would be stamped for the target size.
if the part had a tolerance of +- .005 or more It would be made to be a feel-flush gauge so no measuring tool would be needed.
Having some flat stock on hand and a surface grinder the gauge would take about 1/2 hour to make with no print needed. The gauge print could be made after the gauge is made.
Splitting the part in half would be scribing the part with a surface gauge scriber, and eyeball to the line/nothing fancy.
For an inside radius often I would bevel the gauge corners to facet match the part inside rather than rounding the gauge just to make the gauge quicker to make.
At the big shop, I kept a supply of drops of various sizes and hardness so to have a nice piece on gauge stock on hand. I kept a bottle of gun blue and would blue/black a gauge so only the gauging surfaces were bright so to help the operator understand the gauge function. Engineering often would draw-up after the gauge was tried.
Often I would use a parting wheel set to the go-side of center and so be able to hack up hard stock as fast as one could saw mild stock.