In case it helps, here are pictures of the PSC part (plastic) on an Atlas GP35. P&D Hobby Shop is a good on-line source for PSC parts.
Atlas also sells a part for this type air conditioner. The Atlas part has a slightly lower profile. If you go looking for it, best check the GP60 parts diagram for a part number. It's oddly labeled on the Atlas parts list ("GP60 roof top part A or B" or something like that). That's the one visible in the third picture. I've used both and in the future will simply use the PSC part. If you have an MTH engine with one of those roof-top strobe beacons in the way, the PSC part is a better choice because you won't have to remove or grind down the strobe. The PSC part will simply cover it.
RM