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I have tried numerous times to make a working engine. First failure was free piston nasa type, then pringles can metronome ( i did get it to work with a water piston though) then a couple tries at manson. Mostly failures.
Here is my latest idea, shooting for an LTD version of the manson. I need to be successful, my confidence is very low with all these failures any help will be greatly appreciated. The sides of the displacer cylinder are some non heat conducting material. The displacer is styrofoam with pockets of steel wool.

The idea is clean, logical. But I feel (it's only a feeling, I'm not engineer, neither I never build anything) a lack of heat exchangers. I'm wondering about using the system that is in the photo. It's easy to make with brazed copper, and it's easy to carve fins into the styrofoam.

Im no expert on this by a long shot, but i tought the displacer had to displace or move air. Allow air to pass around etc.

The displacer must also act as a thermal switch, the goal is to change the volume of a given mass of air in the minimum time, heathing/cooling it. I belive that having the two heath exchanger alternatively "switched on/off" by the displacer will benefit the overall efficiency.

What about ratios of displacer to power piston and the size of the ports? Are bigger ports better? I cant even get it to run at all. My first attempts were with rubber diaphragms which didnt work for some reason , so i thought i would try a piston instead.

I ve also seen the port inside the displacer cylinder on the hot side and on the cold side. Which is better?

About the ratio between the displacer and the power piston, it depends from the ΔT but the bigger the better IF the displacer is really lightweight. For this reason the place where you put the regenerator is wrong. Better around the shaft/pipe. I believe that the most critical hole is that on the tube, I don't find the word in my brain (I'm still Italian) but I know why is needed. Well, this hole will be the real problem, so be prepared to trial/error work. I am an artist, not an engineer. But is logic.