Hi team
I am trying to attempt if I could DIY install an aluminum sliding door to my alfresco to enclose it.

Details:
-> house is on bricks for the pillar and sides
-> foundation/ground is a composite decking
-> Size of 1: 2540mm width | 2465mm height
-> Size of 2: 3130mm width | 2450-2470mm height
-> I can get help with carrying this/manoeuvring around
-> Unsure if I want 2 sliding doors but at least I want it on #1 for sure.
Questions:
1.) Common sizes of aluminium sliding doors are 2.1m height x 2.4m width. How can I fill the gaps of those?
For example, I have a 140mm width gap (70mm each side). Do I just get a 70mm width h3 treated pine and bolt it to the brick so the sliding door can be fixed there?
What about its height? I have around 365mm that I need to fill out. Do I just create a small frame for it like this photo?

2.) If I were to create that frame, what sort of items would I need to cover it? Do I have to plaster it or can a flashing just do?
3.) Is this the best way to install this? Any other tips?
First time hands-on DIYer, long time lurker. Is it worth DIY-ing? Thanks!