No offense, but that seems to be a silly attempt. ND filter are neutral in nature, your tinted acrylic most likely are not manufactured with this property in mind. Neutral means it does not change the hue of light going through it, at least it's intended to be so, though some have reported ND filter gives a colour cast. Also ND filters are made of glass not acrylic, your acrylic is not as clear as what glass can achieve, the amount you stop down does not necessarily coincide with the f-stop terminology used in photography for easy calculation and compensation.
Some people have tried using welding glass before, the results are ok if you want to shoot in pure BnW, otherwise will have very nice tint.
Someformofhuman here use acrylic before, you can ask him, from what I see the results are not so good, but will do if you really got no budget. Personally, if I have the money, I'd just buy the ND filters.