Starboard cut dimensions & color suggestions

[DrakeEnglish]

I purchased a used Red Sea Reefer 425 XL and the tank dimensions are 48 length & 23 width. I am purchasing starboard for the tank and the website will cut to my desired specs and was curious of what measurements I should submit, if I need to take a little off, etc. I am not planning on using silicone on the sides as the tank will be moved in 1.5 years once my house is done being built.
Also, I am leaning towards going with the color gray. What do you guys suggest? Any pics would be great. In between gray and white.

 

[Saltyreef]

I used seafoam color king starboard from tap plastics to mimic sand color.
I had it cut 1/16th shorter than my measurements on the long and short edge to help with it not getting stuck. It was a perfect fit.
It naturally floats but the rocks will help weigh it down. I "siliconed" mine down even though the silicone doesnt really stick to the starboard it held it in place

Just be aware it doesnt stay that color forever!