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Repair of Steel Pool filter - 3.5" hole

Q. I need to repair a drain for a 6' steel pool filter filled with sand and gravel at the bottom. the old drain pipe has corroded, fell off, and left a 3.5" hole 8" from the floor. Welder refuses to weld in this tight space.

Thinking about using a Stainless 4" to 3/4" reducer plus a 3/4" tee. The straight part of the tee gets a 3/4 plug with a hole for the jack screw all the way through to the inside of the filter and is held in by a Tee bar. After gaskets and silicone caulking and tightening the 1/4" SS rod- fill the tee's with caulk too. Once dry, screw in a 3/4' valve (probably SS) for winter drainage.

Problem - 4" to 3/4" SS reducer is $$. Thinking of using a 4" aluminum round bar stock cut to 1.5" thick (easy to work with AL) instead of the SS reducer and tapping a 3/4" NPT in the middle.

Now with Chlorine (Sodium Hypochlorite at up to 5ppm but usually at 1ppm), aluminum and SS or Brass valve - what are the galvanic no-nos here?

Mechanical Engineering struggling with a combined chemical engineering and material engineering problem. p.s. I work pro bono for this non-profit summer children's camp. I met my wife in this camp as a kid and feel I owe them. Thanks

ANDREW STEPANOFF
Children's non-profit summer camp - Monroe, Connecticut USA
April 10, 2018