You don't want fish! Make a wildlife pond, they're much more interesting
You have to be aware that ponds, or even baths, are not allowed on some sites. Check your tenancy agreement first
I made mine with a scrap of butyl I'd got. It's neglected at the moment but it looks well despite my not going near it, other than to mow around the periphery. I did have a solar fountain on it but that soon conked
What I would say is that make sure it's level. I made mine only to find the water running out at one end as I'd not levelled it properly. No great problem as I just ended up with a boggy bit at one end which is quite desirable for a wildlife pond
Just bear it in mind is all I'm saying