Here is the XML I am using. Note that CustBlock in my original post is really 'TargetDetector_cf'.
<?xml version="1.0"?>
<block>
<name>TargetDetector_cf</name>
<key>a1System_TargetDetector_cf</key>
<category>a1System</category>
<import>import a1System</import>
<make>a1System.TargetDetector_cf($samps_per_sweep, $num_sweeps, $frame_size, $rang_max, $v_max, $rng_nfft, $dop_nfft, $num_peaks, $num_tgts)</make>
<callback>set_samps_per_sweep($samps_per_sweep)</callback>
<callback>set_num_sweeps($num_sweeps)</callback>
<callback>set_frame_size($frame_size)</callback>
<callback>set_rang_max($rang_max)</callback>
<callback>set_v_max($v_max)</callback>
<callback>set_rng_nfft($rng_nfft)</callback>
<callback>set_dop_nfft($dop_nfft)</callback>
<callback>set_num_peaks($num_peaks)</callback>
<callback>set_num_tgts($num_tgts)</callback>
<param>
<name>SampsPerSweep</name>
<key>samps_per_sweep></key>
<type>int</type>
</param>
<param>
<name>NumSweeps</name>
<key>num_sweeps</key>
<type>int</type>
</param>
<param>
<name>FrameSize</name>
<key>frame_size</key>
<type>int</type>
</param>
<param>
<name>range_max</name>
<key>rang_max</key>
<type>float</type>
</param>
<param>
<name>v_max</name>
<key>v_max</key>
<type>float</type>
</param>
<param>
<name>RngNFFT</name>
<key>rng_nfft</key>
<type>float</type>
</param>
<param>
<name>DopNFFT</name>
<key>dop_nfft</key>
<type>float</type>
</param>
<param>
<name>NumPeaks</name>
<key>num_peaks</key>
<type>float</type>
</param>
<param>
<name>NumTgts</name>
<key>num_tgts</key>
<type>float</type>
</param>
<!-- Make one 'sink' node per input. Sub-nodes:
* name (an identifier for the GUI)
* type
* vlen
* optional (set to 1 for optional inputs) -->
<sink>
<name>RawFMCW</name>
<type>complex</type>
</sink>
<sink>
<name>Rx</name>
<type>complex</type>
</sink>
<!-- Make one 'source' node per output. Sub-nodes:
* name (an identifier for the GUI)
* type
* vlen
* optional (set to 1 for optional inputs) -->
<source>
<name>range_est</name>
<type>float</type>
</source>
<source>
<name>speed_est</name>
<type>float</type>
</source>
</block>