As well as the mechanisms for adding tension, you will want to think about what other mechanisms your loom will have to make rug weaving faster or easier. These mechanisms include shed sticks, heddle rods, shafts and beaters.
My rug looms have a shed stick and heddle rod, which helps raise and lower the alternating warps to make weaving easier. You can pass the weft across the loom in one smooth motion, rather than threading over and under each warp by hand.
A similar mechanism exists on hand looms. Here the warp is threaded through string or metal heddles which sit in frames called shafts. The shafts are connected to the treadles under the loom. When you press down on the treadles the shafts are raised and lowered, raising and lowering the warp with it, and opening a space for the weft to pass through.
Finally as I mentioned earlier, hand looms come with beaters. The beater is a swinging frame that contains the reed. This keeps each warp in place and can also be swung forward to beat the weft down firmly. But I find even on a loom with a reed, I still end up using a comb or hand-beater to make sure my weft is really firm.