Capping a double skin garden wall cheaply

[theSouth]

I've built about 35m of garden wall, about 600mm high. It's double skin with overall width of 300mm for the two walls.

I want to render the whole lot but I'm wondering how best to 'cap' the cavity between the two walls to take the render. The walls will be like those rendered white square topped walls you see in villas abroad.

I have thought of several options, but none of them are that appealing:

1: fill the cavity with concrete
2: buy some cheap 300mm wide capping stones and render over them
3: use some old internal floor tiles I have left

Is there a better cheaper/quicker option?

 

[Deluks]

Wrap the lot in expamet? Not necessarily cheap though.

 

[noseall]

Would 300mm x 600mm concrete slabs not be cheapest strongest option? They would accept render and take knocks.

 

[theSouth]

thanks for the suggestions

I considered off-the-shelf concrete capping stones, but this worked out a lot of money for the number I needed. I then thought I could just cast some myself for a lot cheaper. still might do this.

on the other hand I might just fill the void with concrete since I don't need to worry about moisture travel.

ideally I was looking for a strong-ish product like cement board I could cut down to size and use to bridge the gap and to take the render, but I've not found anything suitable

I guess most people must fill the void or simply cast the whole wall in concrete with shuttering rather than built two skins of block