you shouldn't have to score the plexiglass. The aquarium sealant is pretty strong stuff and should hold ok when it's cured, although it might be a good idea to degrease the edges of the plexiglass to ensure good adhesion. work in a well ventilated area, as the fumes from aquarium sealant can be quite acrid.
As a rough guide, (and if I've done the maths correctly) a tank approx 10" x 5" x 5" should give a capacity of nearly 2 US gallons. of course, you can alter or interchange these dimensions to fit your prop
Assemble the sides and base of the tank using sealant and tape to hold the pieces in position, and then, when that's dry, run a bead of sealant along all the inside edges, smoothing it along with your finger.
Ensure that the sealant is fully cured before filling to test. (you might want to test it outdoors..
just in case).
If any slight leakage is evident, mark with a marker pen on the exterior surface, and apply more sealant as necessary. (the tank must be thoroughly dry before applying sealant).
Best of luck.
