I can't remember ... you're doing the 1/350 Tamiya kit, correct?
What about just getting a long piece of corrugated cardboard from a box? You can rough out the hole and then use props to hold it at the correct waterline level as you fine tune the hole. If the cardboard sags too much, staple it to a couple of lengths of 1x2 furring strips or the like to make sure it stays horizontal from side to side.