I'm not very expert, but two years ago I also thought about building an etalon in my home.
I had two optical windows calibrated with correction on all sides of lambda/20. I thought about aluminising one side of every window with 95-99% reflection.
I did not have problem doing this, I asked an optical company, but I also have an aluminizing bell in my institute. The high reflection is necessary because the "finess" is greater the larger the reflection of the two inner sides of the etalon.
The biggest problem came out when I verified that a lambda/20 correction is not enough to get an etalon. At least lambda/100 is required.
I have confirmed this by using this software
https://lightmachinery.com/optical-desi ... -designer/
With lambda/20 the curve is almost flat, with almost zero "finesse" ...
If it does not work with lambda/20, it will work worse with two commercial filters.
This commercial filters are not better than lambda/4 .. A few months ago I did a test of about twenty quality filters , 2" and 1 1/4"(Baader, Schneider, B+W, Hoya and kenko) and I verified that they are almost all lambda/2. Only one Kenko was lambda/4 (casually).
This is the state-of-the-art value of the optical correction of good commercial filters (in the past I had tested some Chinese economic filters and the quality was much lower, about 2-4 lambda. 6 Paolarized filters of 67 mm marked Panagor had a shameful quality and the thing was strange because since panagor is not a bad brand .... The poor quality was visible with the eyes looking at the view through them. In practice, they were made with glass of windows !!!). Polarized filters have much lower quality of lambda/2 (Baader included, at least in my two Baader filters).
This does not mean anything because this correction is sufficient in the "normal" observation, also because normally the optical beam does not cross the whole filter (when the filter is inside the optical path), but only a part and, for this part, the correction is actually lamda/4 (except for polarizing filters).
So ... unfortunately it is not possible to do an etalon in own home ...