With details like this the question needs to be asked, "Who will this effect and how much will it effect them?" Go with the magnets but do not make it so concealed that you can't find it in 10 years.

Interesting project. Rare earth magnets can be attached to wood and are incredibly strong. I used 3/4" magnets on a cabinet and wish I had used smaller ones. Magnets could be used to construct a tongue and groove panel that matches the rest of the sauna. Another option would be to use a catch or latch. I also wonder whether you should use a corrosion resistant metal for the hinge and closure hardware in such a high humidity environment.

The only way to conceal it is to stagger the joints. I have an access panel in a tongue and groove ceiling and just did the standard board replacement method of cutting off the bottom of the groove on the boards and installing them permanently. The boards can be removed and reinstalled with no damage and I didn't have to think of a creative method to install them. Similarly, this is a situation where the removal is only anticipated in a motor failure, so it is not like they are going to be removed and reinstalled a lot.

However, my baseboards in the theater need to be removable for wiring access and I installed them with these.

Thanks all. They must meet code for an 'easily removable access panel' so I'm not sure that the cutoff tongues would work though from a practical standpoint given the once every 2 decades access that'd be a great solution.

Concealed might have been too strong a word. They don't need to be totally hidden but just blend in as best as possible. So having the panels square/rectangular rather than staggered should be OK.

Inconspicuous