I have not tried the a/c in the companionway, yet. Several other boats I noted have made hatch boards fitted to small window units. I am leery of the companionway being too narrow to accept the width of the a window unit. Also, I am not quite as agile as I once was, and that first step down could be a real lulu. For several years I successfully used a small portable air conditioning unit. I didn't have the table onboard, so it stayed tucked against the port bulkhead out of the passageway. Being an older unit, it pulled about 12 amps running which was fine for the circuit. At night it would almost put frost on the bulkheads. Daytime was a different matter with the sun beating down until about 4pm. The deck would become heat saturated by about 1130 and the temperature difference would noticeably narrow. However, it was still cooler and dryer. You can still find the old "over the deck hatch" portable a/c units. They were heavy as lead and usually needed 2 people to move. On that note, I have seen one gent build a wooden 90 degree hood for his forward hatch. He mounted a small window unit to the assembly and it sat on the fore deck over the hatch.
I hope my ramblings helped. Good luck and it will be interesting to see what you find works best for you.