Introduction and question to the aperture of scopes (1 Viewer)

I am tempted to give it a try. The splittable tube in the TLAPO804 should make it easier.

Any requirements I should have on the magnets, by the way?

I just used two little magnets out of cheap bit holders. You can get stronger ones though, and it wouldn't hurt. I would keep the one inside as small as possible. Careful! They are surprisingly easy to lose! I would keep the outside one on the inside of a band of some sort, something that moves easily but also keeps it in place.
The outside diameter of the larger iris is 76.5mm. My tube inside is about 84, so there is not a lot of room to play with. Your tube is no doubt larger.
What I can't tell from the pictures is whether the adjustment is done by sliding or turning the lever. I have looked at a bunch of pictures on ebay, and the larger ones all seem to be turning, so you would have to drill a hole in the tube once you determine the exact position with cutout baffles of the right diameter depending on which one you get. It would in the end be far better as you could adjust it very easily and precisely.
Edit: Looking at more pictures on google images I would say they are more likely to be of the sliding type.

It just occurred to me there are light meter app for smart phone, so no need to rely on the 1/3EV resolution.
https://play.google.com/store/apps/details?id=com.tsang.alan.lightmeter
These are for Android, similar apps should probably be found for iOS.