I followed this thread on reefcentral too. After taking the DIY calcium reactor MACO course, I'd offer the following unsolicited advice!

1. Use 3/8" cast acrylic, NOT extruded. Cyro GP would be the best, probably. Use 3/8" because the square design is not nearly as strong as a cast tube would be, and 1/4" might cause problems down the road.

I would also consider reinforcing your seams with weldon 40; that's the 2 part stuff. If you just use weldon 4, make sure your edges are as close to perfect as you can get them. (i.e. router/shaper finish) Use .001 steel wire as a spacer to get a good flow of weldon in the joint.

2. Go a bit higher in your circulation pump than you would; maybe a little giant or iwaki; something that will really keep water moving to avoid dead spots.

3. Please let me know how it works out. We actually discussed this design during one of the MACO chats as part of the course. The instructor, who's been designing and building reactors for many years, thought that the square design was risky due to the pressure that can build up in a calcium reactor. I know, theoretically there's no pressure, because the in/out flow is the same, but it does happen; maybe the C02 gets a little high, something gets clogged, etc....

Good luck. My reactor is finished; I'm just waiting on the pump which I ordered about 10000 years ago, it seems.

Matt