Reddit mentions of Behringer V-Tone Acoustic Driver DI ADI21 Amp Modeler/Direct Recording Preamp/DI Box,Burgundy

I can't speak to your insturment specifically. I did however, do a lot of research into acoustic amplification trying to make a nylon strung acoustic guitar work with Rocksmith.

You can get a stick-on Piezo pickup relatively inexpensively from Amazon.com. My experience has been that an expensive pickup won't significantly out-perform a cheaper pickup. Start inexpensive and see if the pickup is going to work before throwing a lot of money at it.

As you already know, you'll need a pre-amp to bring the volume up to a reasonable level. I bought this Behringer external pre-amp from Amazon relatively inexpensively.. This setup is enough to bring the stick-on pickup within an order of magnitude of any other instrument at your gig. You can then run it through an amp or a PA.

Be aware that a Piezo pickup will not produce the deep full sound you are used to with your upright bass. Piezo pickups will tend to produce really wonky response curves with a bit more of a synthetic tone than your instrument produces naturally. They work by detecting the vibrations of the body, not of the tone actually coming out of the bass. They usually pickup string vibration more than acoustic vibration.

If you want to capture the natural tone of your bass, consider using a Microphone pickup. A mic pointed at the sound-hole of your bass can produce very good sound, and is a very common way to amplify acoustic instruments. In fact, this seems to be the most common way to amp an acoustic bass.

A clip-on mic like the one I just linked will help produce the most consistent sound levels as your instrument moves around. But you can use a conventional stand-up mic as well.

A mic will be sensitive to feedback, so properly testing your environment and ensuring that there's no feedback is critical. This will still be an issue with Piezo pickups as well, though it won't be as much of an issue.

TL;DR: Mic your bass.

I would start with the Cheap Dean Markley if you are experimenting. Piezo pickups like to see a very high (~10Ω) impedance, the difference in response when using a proper DI is profound. Links below.

Dean Markley: https://www.amazon.com/Dean-Markley-DM3000-Transducer-Acoustic/dp/B00542YQSQ/ref=sr_1_1?s=musical-instruments&ie=UTF8&qid=1497045344&sr=1-1&keywords=dean+markley+piezo

Cheapest ultra-high-z DI I'm aware of: https://www.amazon.com/Behringer-V-Tone-Acoustic-Modeler-Recording/dp/B000KITQK2/ref=pd_sim_267_2?_encoding=UTF8&pd_rd_i=B000KITQK2&pd_rd_r=CF0BQGFFMPZTJ0TVM3W4&pd_rd_w=jRvQb&pd_rd_wg=tR8Rv&psc=1&refRID=CF0BQGFFMPZTJ0TVM3W4

An inexpensive solution that I have in one of my ukes is
this Artec piezo mounted under the saddle paired with this endpin jack. As you can see the components simply plug into each other. This is a passive pickup so it will benefit greatly/require an external preamp. I use the Behringer ADI21.

I've also installed the JOYO JE-62 in three different ukes for friends. It can be found on eBay from various Chinese vendors. It comes with a flexible wire piezo for under the saddle but I replaced it with the Artec piezo. This is an active unit with a built in tuner.

On the higher end I've installed MiSi Acoustic Trio pickups in two of my ukes.
All are good solutions for different budgets.

Is this a good option for a cheap 'preamp' from an under soundboard passive pickup to a small amp, just for insode use, not gigging or anything?
https://www.amazon.com/dp/B000KITQK2/ref=cm_sw_r_cp_apa_5QHHzbWJH1P7Q