For the headache and cost it's probably best for you to go with a new microwave. I looked up the dimensions of your Magic Chef Model MCO153S and the closest matching option we have is the Greystone Over the Range RV Microwave # 324-000135 since yours is overall 29.9 inches wide. view full answer...
The tray on the MORryde RV Generator Slide Out Tray # MR69RR measures 14" long, 21" wide, and 2-3/4" tall.
This seems to be the closest we offer to your measurements, and can support the 250-lb weight you mentioned as well. view full answer...