Assuming that by 'removable' you don't actually intend for this to come out every other weekend, more like every 2.. 5 years.

In which case, you don't need something easy to see, hinged, screwed  or mitred [which will scratch the wall as you try to lift it straight up anyway].

What you need is decorator's caulk or even silicone. [Silicone is best, but you need to be painting white, as it's tough to paint over].

Once your skirting is in, I'd assume you will caulk the top edge & corners right round to make a better line for decorating. So, just extend the idea.  
Cut your piece of 'removable'. Sand or plane the edges so you won't have a fight to pull it out again. You could even slightly mitre it 'the wrong way' so it won't wedge in the gap. Fit it not quite tight in line with the rest of that piece. Nail or 'no more nails' the rest & just place that piece in.

Caulk the top & the sides where it meets the rest of the board. Paint as the rest.

When you need it back out again, just run a Stanley or modelling knife through the silicone/caulk & out it comes.  Re-caulk it if it needs [with silicone you can often get away with pushing it back into place & the join just vanishes.  
I have a similar structure in a tiled area over an enclosed toilet cistern. The joins never *absolutely perfectly match*, so though it goes unnoticed by all but myself, I can easily find it every time I need to be in there.