Replacing Supply Air Plenum Box

I need to replace the supply air plenum (vertically mounted) that connects the air handler to the main supply duct. The existing one is 30 years old and the flexible boot has deteriorated. When shopping for a replacement duct, do I look for one (or have one made) that is exactly the same dimensions as the outlet on the air handler or slightly larger so that it fits around the opening? The drawings indicate an opening on the top of the air handler of 17 1/2" by 11". Also, do they still make this type of plenum with flexible boot or is everything rigid these days? I like the idea of the flexible boot because it will be easy to install because I won't have to mess with the air handler or surrounding ductwork to get it in place. If rigid is my only option, how would I go about installing it? Size it slightly shorter then wrap something around it to seal the gap?

 

If only the flexible joint has deteriorated it would be foolish to replace the entire plenum. Most plenums are field made and are custom to the installation. Sheet metal work is expensive so anything that you can DIY is money in your pocket.

Here is a site that sells flexible joint material by the foot. Most other sites require you to purchase a 100 foot roll. You might find the material locally at a good sheet metal shop or a place that installs furnaces.

ComfortGurus.com: DuroDyne MBX4-100 - Black Excelon® Flexible Duct Connector (1 ft.)

(I have no connection to Comfort Gurus and have never purchased from them. Caveat emptor.)

 

I can likely reuse the existing plenum, but am preparing myself for the possibility that I'll have to replace it. I'm trying to determine the best way to level my air handler and it looks like I may have to disconnect the supply plenum in order to maneuver it. At least that will make it easier, I think. I may be able to wedge something underneath between the return box that it is sitting on and the bottom of the handler. I gave it a good push/tug the other evening though and it didn't budge much, presumably because of either the supply plenum or the return duct connected to the return box. Doesn't look like the air handler is secured to the return box--I believe it is simply resting on top. Is this typical? I'll post a couple pictures later this evening.