I have a pretty high levels of radon in the basement (~400 Bq/m3 when windows are not open), so I want to seal obvious entry points, one of which is what looks like a 10 inch weeping tile cleanout access point (see photo). At the moment it is covered by a plywood circle cutout. I dropped radon detector in there and it shows 900 Bq/m3 after a day.
I expect a lot of radon is seeping from there, from the sewer drain or from another 10 inch hole I haven't opened yet.
The question is, how do I air-seal this access point but still keep cleanout accessible and level with the rest of the floor? (it is kinda in the middle of the room).
I don't expect cleanout to be needed often (?), so at worst a permanent but removable cap siliconed in would do. Ideally though, there is a sealed access hatch made for the purpose that I can install into recessed concrete or something, but I'm having trouble googling for that.
Update: I have 3 of those cleanouts actually, after digging into the gravel for a bit it seems like it is gravel all the way down (and around). I think that's what slab on grade with wiping tile looks like?
This is 1995 house in Canada for reference. No sump pump or hole for it.