Welcome to the Forum!

Sorry to have missed this til now!

What you are looking for is going to be a liner 'receiver' or 'track' for a cantilever deck.

It looks like your corners were done with the stuff I'm talking about but the straight walls were done with a kind I've never seen ... (on second look, it seems that it might be all the same, as I said - I've not seen this style reciever before)

See what you can look up tomorrow and post back on Fri. (so I'll be reminded) I'll tell you what should work or try to do my own search over the weekend (*I'm NOT very good with using the search functions on a computer )

Try this for a start.