I've seen this black, shallow pool type structure in a few videos, including

Bri and Zoe Page's Formula Demonstration Backfires https://umd.net/forums/topicid/1019502

It looks to be a black plastic rectangle, with inflatable sides, about three inches high. Like a paddling pool, but plain and very shallow.

Can anyone tell me what this is called? I'm think it might work for what I need, but I don't know what it's called to search for it.

Thank you

BTS

Yeah it's a black inflatable pvc play sheet

It's available from quite a few suppliers
here's a link below

https://www.amazon.co.uk/Fist-Inflatable-Play-Sheet-Black/dp/B073Z2M5XY

It measures 187 x 156 cm

I don't actually own but I would love to know if there is one like this but only larger say 2.5 x 2.5 metre

Thank you! I was looking in the wrong places, thinking it was something industrial I was searching on Screwfix.

I know Screwfix do a heavy duty black plastic membrane sheeting which you could use in conjunction with foam swimming pool noodles to make a 'homemade' version of the playsheet tailored to your size needs
Something I might try as well

Just to confirm, yes, they come in different sizes and Amazon and other retailers sell them. I happened to have bought a king sized one, to fit the king-sized bed. They also come smaller. Fairly easy to fold up after use and carefully carry to a shower or outdoor hose for clean-off.