I know PeteLockwood can be harsh with his answers but I have to agree with him....the poster was looking for answers base on his suggestions.
Whilst I agree that Rays do not really need the height, mine regularly climb the glass and this is common with Rays, if fully grown some Rays might not be off the bottom of an 18" before they touched the top.
More importantly i think the proportions would look better in 2 ft high and whilst rays need surface area I think that they are more interested in space than swimming lenghts therefore width is the limiting factor i.e. 10 or 12 makes next to no difference and your width (front to back is already set) therefore my long answer is 10X4X2 would be the tank that I would buy.
Thank you for your opinions! I have found that EGON was right and 12 foot acrylic is almost non existent! There fore im stuck with the 10 foot tank. Im still debating height I have a 6x4x2 now and i put a tape on it and at 6 feet 18 inches isnt much different but when you add 4 feet on it could make a huge difference! I will save a good chunk of money with 18" but itmight look silly
Concur on the 20" height option, possibly even 21" will be the same thickness acrylic as the 18" and cheaper than going to the full 24". Contemplating a 10'x4'x20" myself.
I believe 10' and 12' acrylic sheets tend to be 'custom' sizes already so anyone that can do 10' should be able to do 12'. You might want to look at http://www.aquartaquariums.com/ too, they'll probably quote you within 24 hours and can ship.
I've been talking on and off to a local acrylic fabricator and might go that route rather than a specialist aquarium builder for further cost savings.
Thank you pete I have contacted them also for a quote so far I have 5 quotes! And may the best builder and reasonable price build this tank! I have quotes for both 10 and 12 foot tanks!