I used my Prodlin original single LNB for a long time on 125w for the PBS stuff, all on one polarity like yours. I eventually modded the mount to bolt up to a Invacom dual polarity LNB, re-using much of the original mount parts. Here are before and after photo's.
On some models you can remove the transmit module and find a square bolt pattern on the waveguide that another single LNB can be mounted in place for your second polarity.
Here is a similar example.
On some models you can remove the transmit module and find a square bolt pattern on the waveguide that another single LNB can be mounted in place for your second polarity.
Here is a similar example.