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Print one latencytester-top without supports and one latencytester-bottom with supports. Latencytester-top can be printed with support material, but the logo on the front may not look as nice.
Glue the (already assembled) sensor board into the bottom of the latency tester bottom. Hot melt glue works well for this. Rotation is not critical, though you may have to twist the cable connecting the two boards a little to get the mounting holes to line up.
Press the main PCB into the top of the latency tester and screw it all together with three M2x6 machine screws. M2x5 up to M2x10 will also do.