Have you been there long and has there ever been need to access it? And is it for just your property or a communal drain? That may limit/effect what you can do. If it's just your own and never been an issue, you could consider getting it moved to somewhere less intrusive. Not a cheap option, but worth considering. Or simply blocking it off and filling it over. The obvious issue being if you ever need to unblock it, it would mean a lot more work.
Hard to see the whole garden, but is it too far away from that border to incorporate it, or would that overwhelm the lawn area.
Most times, people just plonk a planter over them.