Some things to keep in mind and think through beforehand could be:
Needed opening height and width before and after the speaker are put in place in your flush mount. To push it in isn’t necessarily the biggest problem. To get it out again, for whatever reason, can be. Figure that out before you construct final baffle wall opening. (My speaker weighs “only “ 44,5 kg / 98 lbs and there is small distance between speaker bottom and the floor where a ”rake” can be inserted to pull it out. Easy to write, harder to do ... An “oversized” opening makes it easier of course but the gaps around should be covered up.)

It might be a good idea to have the baffle wall hole opening up at an angle backwards, instead of a 90 degree corner. -In case you find out you need to rearrange the speaker angel inwards / outwards towards listening position. Change of listening position length wise in the room would probably need some adjustment for the speaker angle.