Hi Dean,

There's pro's and cons to both portrait and landscape designs. Landscape is good for reasons like only having one long screw on Y axis rather than 2 long screws on X axis which is a problem with large format machines, space saving and easy loading are just a bonus IMO.
The down side is that you need to really beef up the gantry and your choice of 45mm profile fastened to a single plate will leave the machine very weak IMO.
Nothing wrong with the format great for light duty wood use but I would seriously look to beef up the gantry.

If you must use profile then why not use 45x90 sandwiched between 2 plates like the pics below.
The wider profile helps with ridgidity and mixed with the extra plate it would be very strong. Plus It has the advantage of protecting the screw with just a slot for the Y aixs ball nut mount bracket which is covered with simple brush draft excluders but still allowing ballnut bracket to slip by.Access is simple by unscrewing the rear plate.
The slot is easy cut into the plate by drilling hole either end then simply running circular saw up each side.

Keep up the good work. .