You are quite right about the clear acrylic - I gave it more thoughts after writing, and yes the edge on show is tricky because that is really all you see when you sit at the listening position. One idea is simply to switch to a white acrylic - no need to paint it as long as it gets a nice fine sanding.
Yes there are three threaded inserts under the speakers. They are supposed to work with the Revel M stands.
Assembly wise I thought it is pretty straightforward - since I can measure the holes under the speaker quite accurately I can have the custom plate cut and all the holes pre-drilled. Then the original metal plate is fixed to the acrylic using countersunk screw in the middle, the pins inserted to stabilise rotation, then the plate is fixed down to the column, and finally the speakers are placed on top and the screws goes in from underneath.
But then I thought..... no matter how neat I can make it, the acrylic will likely be thicker than 5mm as I might have hoped. You are right in that the thickness will likely be more, maybe around 10mm - the weight of the speaker held over 3 screws, which in turn is sitting on top of a small metal plate, yeah better make it beefier, but that will ruin the aesthetics... I kind of dig the look of the speakers sitting atop of a 3mm steel plate...... so minimal. I think it's going to be really hard to beat that.
How much will a powder coated, custom cut and drilled 3mm steel plate cost??
Ah my amp - I got a heck of a deal on it from a shop in Edinburgh. Ex-demo but with very light use. If I had to pay full price I would never be able to afford it
Edit: Regarding the weight of the base - You see the thickness of the base in my photo? Underneath that cover there is an equally thick and solid steel plate. I'll say roughly half of the total weight of the stand is in that plate.