I designed the 4 degree bracket for the GSP 90cm dish about 12 years ago. It was produced to be used with the GSP SL1/SL2 bullet LNBFs. Since those models are now NOS and priced so high, the Inverto BLACK MultiConnect Single LNB would be a suggested substitute.
LNBFs with any scalars larger than the 40mm clamp opening will not fit into the dual clamp. Alternately, it would be possible to remove the plastic housing and cut away the scalar at the adjoining point to allow two LNBFs to sit flush next to each other. I did this in early experimentation and @wescopc has a thread here on the forum on trimming KU LNBFs for 2 degree mounting.
Be sure of the exact skew angles before cutting the scalars. Optimize the skew with the bracket set to the midpoint offset skew (a point that is 2 degrees between the two target satellites). Example: if targeting satellites 97/101, set the bracket angle for 99w to set the two LNBF centerpoints at the correct height. A suitable feed cover would need to be fitted to prevent water from filling the cavity.
Produced step by step documentation for installing and optimizing the dual bracket kit. This material may still exist either in the SatGuys equipment guides or on the SatAV website.