Hi Alan,
an outdoor enclosure might contribute to the health of your animals by providing some „natural“ food, natural lighting and temperature regime.
However, there are some major disadvantages using outdoor enclosures. The most important is the loss of observability and thus the loss of control over the animals. Also a container of the size you describe will not support an ecology large enough to provide sufficient food organisms for your newts. You will have to additionally feed them. It will be hard to check for nutritional status, eggs, larvae, sick or dead individuals etc...
Some experienced keepers even lost their whole stock of one species due to unforeseen reasons in such enclosures.
If this didn’t scare you off, some basic requirements for an outdoor enclosure:
1. The enclosures have to be escape-proof but also have to prevent the intrusion of native newts.
2. They must have appropriate and escape-proof facilities for excess water (heavy rain-fall) in and around the enclosure (drainage).
3. They have to be covered against predators like cats, polecats and the like but also against dragonflies and large water beetles (predatory adults and larvae).
4. Provide shade and shelter.
5. Provide some islands or adjacent areas with hideouts to check for terrestrial animals or potential offspring.
6. Provide overwintering facilities if you’re going to plan to leave them outside during winter.
I’ve been using white 120 l tubs (butchery supply) from March to late October for some of my P. waltl and T. dobrogicus for the last two years. I check the enclosures (or rather outside tubs) at least once a day. I check on the status of the animals once a week by catching some. For reasons of protection and insulation (or rather temperature buffering) I made myself some wooden boxes with lids around these tubs (see picture).
Maybe some more experienced „Outdoorians“ can provide a little more detailed info on the matter and especially on the fire salamanders. Alan, keep us updated on your outdoor enclosure.
Ralf