I use a 13cuft pony. I managed to find a cheap integrated valve/regulator.
For what it's worth, backmounting a pony can be dangerous. Several accounts of deaths directly attributed to a backmounted pony here:
Do you actually see people diving with pony bottles?
I took a minute to read the accounts and I agree with
@saxman242, the incidents were related to training rather than indicating that a back mounted pony if inherently dangerous. In the first case a diver did not properly check that his air was on, this could happen with a back mount single or back mount doubles too. In the 2nd incident a diver did not know which regulator was which. That he did an arm sweep to recover a lost regulator and ended up with the regulator attached to his pony rather than his primary tells me it is likely that his backup was dangling freely rather than being properly secured. Then the diver, when OOA, never switched to the other tank, that sounds like a training issue not a configuration issue to me.
There are practices, and equipment configurations,that you can use to minimize the risk, but not fully eliminate it. For example on my back mounted pony my primary is a 40’ hose wrapped under arm and my back mounted pony is on a short hose that is bungled around my neck (in primary donate fashion). This works well and is one of the techniques mentioned in the thread you referenced.
I tend to think the OP in that thread drew the wrong conclusions. The conclusions he should have drawn were that pre-dive checks need to be thorough and divers should be highly familiar with their configurations and trained to do the dives they engage in. Specifically the OP in that thread concluded that side slung ponies are safer than backmount ponies because tech diver have been using them that way for decades. He attributes success to the wrong thing, configuration vs training.
I would also say that Scuba diving is inherently dangerous, that is why training is required before you are allowed to dive and you have to routinely practice skills to maintain proficiency.