I had to mail order mine, as they don't have it at HOme Depot or at the plumbing pace near me. TAP Plastics, I think was the name of the store. They have an online dept. I ordered #4 and #16. (I think those were the numbers). The has to do with the materials to be welded and the viscosity of the solvent - the low numbers are very runny and can only be used for bonding a vertical piece to a horoziontal piece which match perfectly, which is what you'll be doing. This is the best way. I'd still touch it up with the other, thicker solvent (higher number) on the inner edges, though, to guarantee a watertight seal, especially if you cut the acrylic. Let it cure the FULL amount of time recommended on the labe (I think it's 48 hrs).
Good luck...I'll be doing the same later this year when I convert my 90FW to reef.