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I know that there's a lot of you handy types on here, in fact, i'm blaming you for the inspiration to have a go at this bloody project in the first place, so it's time to pony up some advice.
The mess that i've made so far can be seen below, there used to just be a hardboarded in area where I made the hole, there's a gas pipe running underneath, hence not just screeding the floor level.
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I had planned on sticking a sheet of ply across the front, cutting out a door shaped opening and using the cut out bit as the door. I suspect that it'll look a bit crap though and there's a better way to do it.
Alas, as you can probably see from the effort already, i'm not that handy - what's the best way to go about it?
Cut a hole in the ply and make a little tiny door frame out of beading? I'd
tongue and groove(with a 2x1 frame on the inside to keep it all secured) in my house with some 2x1 for the framework
assuming a rectangular door...
2 stiles (long edges) 2 rails (top & bottom) from 28mm square timber
a lockblock where you want the lock to go, again 28mm thick timber
2 skins of approx 5mm thickness and the right size for the hole. plus any core material you think fit
lay one of the skins flat and glue the stiles & rails to the skin, stiles to the long edges, rails to top & bottom between the stiles. glue the lockblock to the skin in the appropriate place. insert core if used. e.g. 28mm particleboard if you want sound insulation, hexagonal paper mesh if not.
glue the other skin to the rest. clamp together for a suitable drying time.
and then it gets tricky... hanging the damn thing!
We've got a small cupboard like that under our stairs, too small for a normal door. We just stuck a bit of ply on some hinges (9mm I think) and painted it to match the woodwork. Looks fine, does the job.
Its not that big an opening. You could frame and skin with ply or plasterboard leaving the door opening then find a kitchen cabinet door you like the look of, or a door from a nice old bit of furniture and fit that.
Its a small enough opening that your idea of just cutting a door shape out of a bit of 12 or 18mm ply would work fine though
Use a stopped tongue and groove joint with wood glue to reinforce the joints then carry the door on with the joints running straight getting the timbers length shorter to blend with the stairs then fit pre moulded strips that have been through mortise and tenon mitre jointed to finish it will look stunning trust me I'm a bench joiner and carpenter i do this stuff everyday
How do you make a door?
Try converting ajar 😆
make sure you put in plenty of sound proofing so you can lock people in there and no one can hear them 😈
I did a very similar thing a few years ago and cut the doors out of a piece of mdf and used blum concealed hinges. Painted the whole thing with a roller and it's almost indistinguishable from the spray painted kitchen cupboards.
To get a really nice edge on the door I cut it roughly to shape and then squared it all off with a router and then used the router again to add a 3mm curve to the outside edge of the door.
Or you could get the polish to do it
Unless you're actually living in a country mansion set in 15 acres of landscaped ornamental gardens, I'm not sure I'd go to town on an understairs cupboard TBH.
I'd do:
Frame it with some 2x1.
Ply door, cut to shape.
Paint, hinge, add a catch.
Fill with shoes / hoover / dogfood.
Hinge the whole or tallest 2/3rds of the ply, on the longest edge better for getting stuff in and out
You're much less likely to ake a balls of this as you're only cutting one shape to fit the hole (no offence - I'm thinking that your skills are roughly equivalent to mine - although I see you're using a sqaure so you're already a step ahead)