Unfortunately, all band pass filter I have seen, require very professional, square-flange LNB and waveguides, so I don't think you'll be able to get everything together for any amount of reasonable cost
There's two kinds of band pass filters used in satellite industry:
1. Directly mounted on, or are part of LNB ones, these ones are quite rough, and for Ku band they require micrometer adjustment accuracy, which is very very hard.
2. Ones that operate on intermediate RF frequency, which goes from LNB. These are considerable cheaper and accessible.
While your ideas for setting up 2nd dish and subtract it's signal from another is logically correct, technologically it's impossible in given frequency range (both LNB oscillators should be frequency coupled, operate in same phase, length of cables to be same up to 0.1mm and so on).
For some bandpass filters, check this as reference:
_ttp://www.microwavefilter.com/ku-band_receive_bandpass.htm
There are a lot of small screws on it, they all have to be adjusted in particular way to reject certain frequencies.