Hi Ken,

The QRP-Guys 50 CW amp has the low pass filter included with the correct capacitors.  The 50watt amp is designed as a single-band amp to match the single band QCX series of transceivers.  

For me, I am using one of two 100watt rated low pass filters.  One from eBay:
https://www.ebay.com/itm/282860312127
or
https://www.minikits.com.au/electronic-kits/high-frequency/hf-filters-lowpass/eme209-kit

I have both and both have about the same filter response.  I tested them with a DIY 100watt kit from eBay.  I had more trouble building the MiniKits filters as the SMD diodes that had to be installed on the select lines were really tricky to get installed correctly.

Since I started this response I see that Hans cleared it up that the QRP-Guys LPF kits are suitable for 10watts.  I would go with those as the build instructions and support are much better than the other two.  I have also read that there have been some shipments of the eBay kit shipped with incorrect components.

Hope this helps,
73
Evan
AC9TU