What I came up with from an old printer box and some spare carpet and underlay is a sturdy, well insulated and easily cleaned cat cave which should be an ideal den for Ozzy. (The photos make it look like the lining is very thick. It is actually only about 1/2" covering the inside of the box and there is plenty of room inside.)
Step 1: Tools and Materials
The best tool for cutting cardboard, carpet and underlay is a heavy duty utility knife (Stanley knife) with a sharp (new) blade. I also used 'non-solvent' building adhesive for the underlay and duct tape for holding things together while this set. Also, tools for measuring and marking will be needed.
The underlay I used has a non-absorbent surface so will be easy to disinfect and wipe clean when required. I was initially going to use hessian backed carpet but then found some foam backed which I thought would be easier to bend and glue.
Step 2: Cutting and Assembling the Box
Mark and cut around the box at the required height then cut out a section from both pieces for the entrance. On the other three sides, cut a 'V' slot at the centre to allow the ends to flex inwards. This is necessary to reduce the width as one part of the box needs to fit inside the other. The picture notes give more detail on all these steps.
Stiffen up the two box halves by putting globs of adhesive between the cardboard flaps and pressing together. The 'forget nails' non-solvent adhesive I used is great. It grips the two surfaces together and sets rapidly, but as I was continuing with the construction I also used duct tape to stop things moving before the adhesive set. Once the two box halves are firm, slide one half into the other and work adhesive between as many of the surfaces as possible.
Step 3: Installing the Insulation / Padding
Cut long strip of underlay to line the rest of the inside. The underlay I used was 4'6" wide which was enough to use a double layer for the floor. Roll up the long strip (rough side outwards) and feed it through the entrance. Unroll it around the inside to check the fit then apply adhesive and stick each side in turn. Don't press the underlay hard into the corners, but allow it to form a smooth curve to reduce the 'boxiness'.
The underlay I used has a non-absorbent side which will make it wipe-clean in use. The double layer on the floor and a cat blanket inside will give a nice padded floor for Ozzy.
Step 4: Covering the Cat Cave
Prepare a piece of carpet for each end by marking around then cutting. Glue and attach the back end piece.
To get the cut-out in the right place for the entrance, align the piece with the front and then push a safety-pin through close to where you think the entrance hole ends. Lift part of the piece without moving it so you can see how close you are, then push the pin through again until you get it through exactly in the corner of the hole, marking it once you've found the right spot. Repeat for the other corners of the entrance.
Draw diagonals between the points you've marked and cut down these to give lift-up flaps, but cut the lower one straight across so there's no flap. Coat the box front and carpet piece contact adhesive, but leave the flaps. Once the piece is touch-dry, attach it to the front. Bend each flap forward in turn and apply more adhesive than you would normally do to each one, then bend back inside the box and press so it transfers adhesive to the cardboard. Wait until touch dry then bend back again where it will immediately stick to the card.
Cut another rectangle of carpet the width of the entrance and long enough to go from the base up to the entrance and bend it over onto the floor. This nicely fills the indent where the box is only single width cardboard and provides a nice bit of detailing to the entrance. Any loose strands of carpet can be snipped off with scissors. Ideally, I'd like to cover the rough joins around the edge so if anyone has ideas, please comment.
Step 5: Ozzy and the Cat Cave
Now Ozzy is a cat, and it is inherent in cat nature not to do what you want them to. So we put Ozzy's blanket in there and presented him with his wonderful new cat cave . . . and he totally ignored it. Cat treats, feathers on a string and even the ultimate attractor of a laser pointer would not lure him into the box so I'm still waiting for the definitive photo of the cat cave with Ozzy's head looking out. (Hmmm, there's always photoshop . . . ) Until I get the ideal photo, here's a few more of him looking handsome.
Update - Ozzy steadfastly refused to use this for its intended purpose even when I cut a hole in the other end to turn it into a cat tunnel, although now he uses the sides as a scratching post and loves to sit on top and wash himself or survey the garden through the conservatory windows.