I would consider building a slatted locker/shed the size of the base slab. That would cover the pool pump and the doorway, and provide some storage. The doorway itself could be filled in with blue board etc and painted when you paint the rest of the render.

Agree

What is on inside -- I assumed it was the garage , but just reading your spiel I'm trying to work out your prioities ? Do you want it to look 'flat' ( not there ) or do you want it to still have a door and be a storage shed ? If doing it as a storage shed , why are you thinking roller door ? The existing door looks in poor condition, and opens inwards , but if space is a problem , then get your builder to install a new door and make it swing outwards . A roller door is out of character IMO -- you occassionally see them in the toilet block at a park , but not a home garage IMO .

Once you've decided that , then you decide on a cover for the filter . If you need a door , then its going to be a bit chunky . If it was mine , I'd try and do rendered blue board with a sloping roof , in the same tiles if possible , Make it look integrated .

If you didn't need the doorway , then I'd do a window at the top , maybe 750 wide by 250 tall or whatever fits , and take the filter cover to that 1.5mtr or whatever width the slab is , again the same slope and even same tiles as your roof , or paint the iron the same tone if tiles aren't available .

I was just looking for ideas that I maybe haven't considered. I do like the idea of a locker with sloping roof and using tiles the same so it looks integrated. I havent ruled out a roller door as we have a garden shed close by with roller door so I dont think it would be out of place if I matched with the same colour.