wanted-DVBS Modulator

The CDM570 (70 MHz IF) or CDM570L (L band), without Vipsersat IP module option, would appear suitable.  
See https://www.comtechefdata.com/datasheets/ds-cdm570-570L.pdf
With 0.05Mbaud (i.e. 50ksps) you will need a bandwidth of 70 kHz.
Example possible information bit rates are:
QPSK  7/8 Turbo  87.5 kbit/s  clear sky Eb/No=8.3 dB
8-PSK 7/8 Turbo 131.25 kbit/s    clear sky Eb/No=11.0 dB
16-QAM 7/8 Turbo 175 kbit/s   clear sky Eb/No=12.1 dB

( Clear sky = 4 dB link margin above 10E-6 BER threshold )

Provided that you don't have to pay extra for excessive power from the satellite (i.e. disproportionate to the 70 kHz bandwidth), you can achieve much lower cost per information bit if you use the higher order modulation methods, but you do need a much larger receive dish, your BUC should be operated backed off below saturation, the receive LNB should be PLL type and the DC supplies to the BUC and LNB must be well smoothed to minimise phase noise.

Consider also the Evolution PD10 modem
https://www.paradise.co.uk/collateral/datasheets/206099_RevA.pdf
from Paradise (Witham, Essex).
The guaranteed specification Eb/No versus BER is virtually identical to the CDM570.
wxw
If you are not buying the modem for each end I would suggest getting a modem the same as the existing one at the hub.

Best regards, Eric.

The creation of a DVB-S type carrier requires more than just the modulator.  There are significant bit stream construction considerations.

I've had a look for a few more modulators called DVB-S2 which incorporate such extra features.
I've found one that goes as low as 0.05Msps  See here: https://www.newtec.eu/index.php?id=119

Others, which may not go at low enough speed:
https://www.eccincorp.com/ecc9600.pdf ;  (100ksps min)
https://www.commsonic.com/downloads/sd0025.pdf ; (min speed not stated)

Given that you are trying for exceptionally low bit rates high efficiency would seem important - i.e. you don't want to waste too much on unnecessary overhead bits.  The earlier two modems I mentioned would simply relay your data stream - you would need to add any external framing of the data yourself.  Talking to a company like Newtec is suggested explaining your service arrangement.

Best regards, Eric.