Astringency is from pH deviation, not fineness of grind. Big breweries mill to flour and then squeeze the grain against fine filters using air bags to maximise efficiency.
I wonder how much pressure a grain bag can take and how we could best squeeze it.
Consider the kr2040ax from eBay. $17. It's basically an SSR with built in control circuitry, and come s with potentiometer to control it. You'll need a heat sink too.
Maybe your low estimate is leading you astray. Can you really not boil 2.5gallons of sugary liquid on your stove? When i was using stove alone I was boiling around 4gal in 6gal pots. I am boiling 10gallons with 3375W in my insulated keggle. I'd be amazed if stove + 1375w doesn't do it...
I use two elements on 120V, a 1375W and a 2000W. One in the keggle, one heat stick. I bought a kr2040ac power controller ssr to adjust temp, but don't need it. I just turn one element off and the other will hold a boil for my 10gal batches.
if I were doing smaller amounts then the power...
I have about 15 1L EZcap bottles for sale.
I would like to sell the 1L for $3 each
or 6 for $15 + 2 homebrews
or 12 for $25 + 4 homebrews
If you want them all I will get the exact number for you.
I have 2 2L Fliptops. They are the type with metal handles. Cost around $30 new, I will sell...
A silicone washer might well work better. The nut I used (for my heating element) came with a groove that traps the o-ring and compresses it. http://www.bargainfittings.com/index.php?route=product/product&path=36_38&product_id=57
Since you have 4" pipe you could put a coil in it, maybe put a sealed 3" pipe down the center to fill some of the volume, which you could also use to coil the tubing around.