make them as long as you possibly can, can put 1/2/or even 3 beds in them, wrap every joint with plastic to keep rain out,if you wantpoint all joints with cement as rabbits dig in and f**k them. bend them too make sure entrance is not in a direct path of wind. in the bed put one block edge on in the middle as stops wind rattling strait through and then the dog goes round the bed as some dogs if keen can go strait through the pipes and nearly miss something in the bed. key is not to make beds too big,like i said a 3 by 2 slab floor a block and a brick high right round and same slab for the roof