Ok, I've literally THROWN this together for you, just so you get the idea
Just used a sheet that I'm in the process of cutting up for bedding and a very quick stitch (VERY QUICK). And it's on a very small scale - but eh, you know how to do it properly
1) Get yourself 6 squares/rectangles of whatever you're using to make the cube and lay them out as above. I cut a hole in one side for the entrance
2) Attach sides to the bottom square and stitch lid on to one side (I've colour coded it for you - each coloured side stitches to the same colour)
You should have something that looks a LOT neater than that, but you get the idea
3) Stitch again along the coloured lines. Remember - we're making this inside out first, so all the stitching will be on the inside
Ok, and you have something that looks like this - again, this is ROUGH, yours will look neater
4) If this was me, I would attach material loops (poking into the middle right now) to each corner of the top square for curtain rings later. I don't know how you're looking to attach, so I've left that adaptation to you
5) Just turn inside out through the entrance hole. Add nice and cosy bedding inside
Like I've said over and over this took me 10 minutes to do, just to give you the idea. I'm going to try make a proper one when I have the time.
I hope this helps
