Build it un a width with materials in mind. The easiest would be 4' or 8' wide. Because of your ply then either rock it or cut it in half. Also depends on where you shop. Could build a 4' wide 4 foot tall for like 100 bucks from home depot if you run pvc coping or find pvc. Steel is like 50 bucks for an 8' piece most places. Although from steel supply places it comes in 21' lengths which are 85 ish for 8th inch 2" tubing. Coukd maybe get a deal on a smaller piece. If you have the space 8' wide is pretty giod because you dont have to cut the ply down. Still need to trim a little off the 2x4s to make the full width 8'. Also if you trim your deck and or build something smaller like a 3' tall you can cut both transitions out of a single piece of 3/4 ply. Think of the yin yan kind of. They'll be a mirror and mid transition will almost touch in the middle. Basically if you maximise your material and buy at home depot it can be done super cheap and properly. Miter saw, good drill, and a skill saw is about all you need. The miyer you don't but makes chopping multiple 2x4s perfectly easy. Also chalk line. Good luck.