A legit optical spot has five controls;
1. Aperture, to adjust the size of the projected pattern, more blades means smoother edges.
2. Cookie holder, this plate has slots or clips that allow you to swap out the cookies, which are made of stainless steel with various patterns. Keeping in mind you may be able to adapt other brands of cookies to fit yours. You can make your own out of Cinefoil too.
3. A blade slot to slide in metal blades to cut off the edges. This lets you change a round projected circle into a square, trapezoid or with two blades a slot at any angle you want which is what you are looking for.
4. Focus control allowing you to either blur the projected pattern or make it tack sharp. The better optical quality with reduce chromatic aberrations at the edges of the focus point, slightly defocusing a poorer optic can help if you are not seeking a fine detail in the pattern.
5. Zoom, this lets you adjust the size of the projected pattern with sliding internal optics. It can be useful if you need to place your spot at a particular location and change the size of the pattern.
Some of the above features may or may not be critical to what you require but options you are looking at don’t have the blade slot. You can use flags to give you the slot effect, it just won’t have a sharp transition.
Considering Amazon’s return policy and that price, I’d be tempted to try that one.