Volume Restrictions and Tuning Boxes

Last month I drew up a design i thought about for a box for a t212 rockford sub. Then I got interested in the idea of making it out of fiberglass. Im not an expert with fg but im not noob either. I read tutorial after tutorial on fg sub enclosures and thought I had enough to begin, then I realized Id skimmed over some important details. Volume and Tuning Frequency. I am confused on how you are supposed to find out the correct volume to use, i understand using peanuts, rice to get a grasp on what the boxes volume is close to but how do you know what you want in the first place? ((Well this is an edit, I just grabbed the specs on this sub again and it has something that is probably what im looking for; it says Rec Sealed Enclosure - N/A and Rec Vented Enclosure - 2.25 cu. ft./63.72 liters.. Does this mean a properly built vented enclosure would have a volume of 2 1/4ft^3? And so a sealed enclosure has no volume limits?))

Alright, secondly tuning. The subs tuning frequency is 40hz, im having trouble grasping how to confront this problem at all.

Ok and then the issue I see everywhere. This sub weighs a good 60 or so pounds and when its cranked the thing almost jumps around, so i guess my last question is a fg/mdf hybrid box or a completly fg box. I havent used mdf at all, mostly repairs on wrecked fenders, hoods, etc. so im not sure  how good it would turn out, and how stable it will be once its done. But whats easier, stronger, better looking, better performing? If anyone wants to have a look at my plans ask please. So ya any answers or comments would be soo appreciated. Thanks

The specs are saying that using the subwoofer isn't recommended in a sealed box. As for the airspace for vented, yup, your final airspace will be the 2.25 ft^2 after calculating for ports, bracing, speaker displacement, etc.

For the tuning frequency, use a program (or ask someone here) to get you what port size you need exactly for the tuning. What other issues about this is giving you a problem here?

A MDF/FG hybrid box will probably be the best bet. Use the MDF to make your rings and straight braces for the sub. FG will make the box and provide additional support. Get it to at least 1/2" thick on the FG to keep the 60 lbs in place.

I'm sure there are others with other ideas. They are not wrong, just another way of doing things.

Good Luck!