I have a lot of cables, junctions, etc. behind the skirting board I want to be able to access it in the future, without ripping apart the board again. I was thinking magnets, some click holders, or hinges. Or make some kind of doors?
But I have no idea what would be the best solution.
I'd just install with "trim-head" screws. Easily removable, not trying to get absurdly fancy about it. Trim-head because they are smaller and less obtrusive than normal-sized screw-heads.
The "slide in place" suggestion by @Solar Mike could be implemented with mitre cuts at both ends of a section of board (removable - long points to the wall, fixed - long points away from the wall) or via spline joints at both ends. Depending on fit it could either be somewhat fussy or excessively loose, and would be prone to getting jammed with paint.
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. One piece of the top is a removable board, cut out of the rest of the top. the joints around it are silicone rather than grout. 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.
From your description of skirting I am guessing that this is a single or double wide trailer home. If that is correct, for my sister, with the same situation, I cut an opening in the skirting with a fine metal saw blade that was big enough to allow access to the area in question. I then installed a treated wood frame with the end boards buried in the ground for support and connected the skirting to those frame boards. I then made a door out of plywood to cover the rest of the access opening. When finished, I painted all the exposed wood with outdoor paint to match the skirting. Shazam, an almost hidden access door to the underside of the trailer.
