You may have seen the cardboard pyramids covered with some sort of carpet that you can buy in pet shops for kittens to play in and scratch - supposedly anyway, as they're so flimsy that they'd probably be shredded within two days and 10 pounds or whatever they cost is definitely too much for that.

I made my own from plywood, with a square base rather than a triangular one so it would be easier to put together, so the base is 60cm x 60cm, and I think it's 60cm high as well, maybe a bit more. At that size, your Siamese should still fit into it when he/she's grown up! You need four triangles of whatever size you decide on, one with an opening cut in obviously. I actually mitred the cuts when I sawed the triangles out of the plywood and then stuck them together with wood glue. That might not be the easiest way though, as it's hard to cut exactly straight and then if the edges aren't touching everywhere it's hard to stick them together, so you might think about joining the edges by sticking them to a piece of wood inside each corner. In any case if you use glue I recommend that you use some from a glue gun, as it dries really fast and that means you don't have to try hold a very wobbly structure together for ages while it dries as I had to. Then cover the whole thing with carpet, for that I used a glue gun and it worked brilliantly. Just cover the entrance hole initially, and when you've finished covering the pyramid, make a hole in the carpet in the middle of the entrance hole and cut with a carpet knife outwards into the corners (if you have a triangular entrance hole like my pyramid). Then you're left with three flaps of carpet you can glue to the inside of the pyramid, and the edges of the entrance will be all smooth as the carpet goes round and covers them so your cat won't scratch itself as it goes in.

You should sand down the edges in the corners inside to make sure there are no splinters your cat could hurt itself on, and I also stuck a couple of lengths of brown sticky tape down each corner to make extra sure and also so there wouldn't be any small gaps that are hard to clean and where bacteria could multiply happily. You can easily disinfect the whole thing on the inside, and it works as a climbing structure (Koshka loves perching on the very top) as well as a hooded bed. If you put a large cushion underneath it's nice and cosy for a small kitten and when your cat gets bigger you can just put a soft blanket underneath instead so there's more space inside. The whole thing cost me about 8-10 pounds, as I only bought the wood and got the carpet for free from a friend who had some left over, and we had all the glue and stuff anyway. And I'm sure it will last longer than the cardboard one!