I did this cheap & easy with raw netting & velcro, and I've been very pleased with it. It works fine if you don't go through the doors much. Cost well under $100. 1. I bought raw mosquito netting (from mosquitocurtains.com). 2. Cut it to fit across both doors, allowing extra width to fold edges under for neat appearance, & of course some overlap onto the door frame where the velcro would be. 3. Attached 1" velcro patches, 3 across the top and about 4 down each side. (I stapled one side of the velcro onto the curtains with an ordinary stapler - 2 staples on each. Super quick. Nailed their mates to the door frame (eek!) with 3 small nails. Doorframe, curtains & velcro are all white, and it looks surprisingly neat and nice. (I could even white-out the nail heads!) Still, in winter I'll probably take it down, leaving just a few inconspicuous little velcro patches.

Decent quality sliding - or folding sliding doors - will do the job and last more than 15 years. Of course you have to fit them properly for a good seal so choose an experienced fitter, a specialist fitter or buy a door that can be fitted by the manufacturer. As with anything, you get what you pay for. Untreated softwoods are likley to warp over time so treated woods, hardwoods or metal doors will perform well over a longer period.

I have a patio door with a single sidelight the same width as one of the patio doors. It looks like a 3 panel setup but only two of the doors open. I think they are 36" wide but would have to double check. So a bit wider that your existing opening. Looks similar to the door in the picture but i have two doors that open.

French doors usually come "pre-hung" which means they are installed in the door frame already. To do it right you need to buy doors that match the existing opening or alter the opening, including installing a new header and framing for the pre-hung doors, to fit. Altering the doors to fit the existing opening is not a good plan.