On Monday, June 22, 2015 at 2:15:03 PM UTC-7, stryped wrote:
My house was built around 1998 and has the original split system with a carrier 3 ton
condenser outside ... I had someone quote a new system.

The installer made a note that under the house, the previous installer rather than attach a return plenum just ran the return line directly off the side of the air handler.

So, I would like to build a filter housing, return air plenum, and maybe build a supply plenum myself 1st. Then maybe decide if I want to tackle replacing the current condenser/air handler.

I've seen HVAC sheet-metal shops bend up these things. Measure carefully, and
get them to do the work: they're fast, efficient, and already know how.
Check some local stores, see which filter sizes are well-stocked, and use
one of those sizes, but you probably already knew that.