I wrongly write 15.5cm for the inner diameter of the Cheapy ERF cell, it should be 14.5cm and of course, it's larger and will not be a good fit. The gap between the cell and the OTA is 0.5mm which is not too large luckily.
To fix the problem, I will make another ring myself by using thin wooden board to make up the gap. This board is relatively easy to make, and I've a dull black board which shold serve the purpose very well, I'll have to paint the aluminium ring black even if it fits, now the wooden board will do the same job.
It might not be very elegant, but it should certainly be firm and secure.
Originally, the hardest part to make this cell is the three set screws, I need to drill and tap thread to it so that I can fix it on the original aerial lens cell, and I don't have suitable equipments. Now, I am to add a wooden ring to make up the difference, the supporting part will be the whole ring instead of that three screws, so the whole thing will be secure. However, if I were to make the ring from scratch by myself, I will doubt whether the screws will be enough or not for the board itself is rather fragile given the screws are small.
After around 10 minutes word with a hand cutter, I managed to make an ugly ring by myself. By ugly I mean the cutting are not concentric, but since the Cheapy ERF has more aperture than I require, it will be totally covered. I've conducted some test last night to see if it's a good fit, I'd say it's perfect.
First test is to invert the OTA so that the Cheapy-ERF is forward downward, it won't fall and then I tapped it with my hand, it won't even move.
Second test is to pick up the whole OTA by using holding the Cheapy-ERF, the OTA life up and it's still very firm.
Excellent fix I would say.