If you are in Perth where there are very common, it is hard work but the digging is easy due to the sand.
I suggest concrete so you are not doing a very awkward and costly repair in a few years.
A good base of blue metal gravel or even pea gravel (which is easy to get here). Fill the base a little as well and down the sides too if you can. The top of the lid need to be at least 300mm deep but 400mm is better. That way you reduce the risk of having sand blow up through your paving.
I find the ag drainage pipe much better and same depths. I often link them to form a network so overflowing isn't such an issue.
I don't like to join the down pipe directly to the soak well pipe as is done to stormwater over east.
By separating them at the base with a grate of sorts (I'll post later) when the soakwell fills (and it will) you don't get a build up of water pressure forcing out sand through your paving and you don't have your eves flooded.