OK here goes, to start with on our 235S rig, the Dexter Door screen ID and OD measurements look like so.. Inside edge to edge is only 26-5/16"
Outside edge (fits into door frame opening) is at 29" (+/- a smidge)
We bought this from Lowe's;

for;

....
Then... removed the screen door from the rig at the Hinge Points (3 screws at each of the three hinges);

WARNING.....
These are self tapping screws and pretty small so do so carefully or they will NOT hold the threads when you put it back together... to help fight that I added Loctite thread (blue) agent to each screw when putting back together
First I laid the screen protector on the door frame so I could mark WHERE the "legs" of the device fell on the door frame vertical side rails so I would know WHERE to cut my channels/grooves. With that decision made, I measured to trim/shorten the "legs" so that they were end to end slightly smaller length than the 29" OD... basically here you are aiming to remove the thickness of the aluminum screen door frame material... if I recall I targeted for ~3/16" +/- narrower than the OD. SAVE those (or at least one) of the scraps you remove because it can be used as a test feeler gauge when you cut your grooves to make sure they will slide into them. Using a Drill bit that was the SAME diameter as the groove channel in the screen door, and then a Dremel small tube shaped grinder, we cut four slots into the aluminum door frame grooves that matched the size of the screen protector "legs" (this is where you can take that scrap and TEST fit to see if you carved enough out.
In the end I discovered the screen protector COULD be slightly bent in the middle so I MIGHT have been able NOT to have to cut "peel back tabs" on one side.. but since I was TRYING to keep that new protector in proper shape.. I used a small cutter to create a "peel back tab" on the hinge side of the door (figuring that would be furthest from slamming forces at the latch side) so that I could "open" a path for inserting the protector legs..... The idea was to bend the protector as little as possible and then just secure that tab back in place with a self tapping screw;

...
Here is the latch side;

and...
Excuse the extra scratches and bad paint job I have yet to have the time to correct that but do plan on doing so...
For NOW when we want to "lock" the screen door to keep it from accidentally being unlatched/open, we simply throw the door handle closed. The result looks like so;
__________________

Ed & Chris, Empty Nesters (missing Josh & Jaclyn)
along with Blue & Tessa, Cat Explorers!
2020 Rockwood Roo 235s
2019 F-250 Diesel 4x4 Super Duty