You can make anything if you put you're mind to it! there was a genuine one for victa's for many years. Most people just use a bit of string to hold the flap open a couple inches.
If you want to spend the time to design it up go right ahead! Ultimately it depends on what you are trying to achieve, mulching is better achieved with a block filling the chute so the clippings are better cut down, and if you are cutting long grass, as i said above the same effect can be achieved with some string or wire on the door.
Another alternative would be to look for an actual side throw rover from the 70's, they pop up usually in running order for around $100 and are brilliant machine for longer grass you don't want to catch.