I have a 2m x 0.5m corner plot with a 30cm high stone wall along one
long side and a 60cm high stone wall along a short side, with the
other two sides open.

This plot used to house 3 diseased roses which I have removed, along
with all their soil (grey, mostly clay and stones, 30cm deep) and the
subsoil (brown, almost pure clay with rocks to a depth of another
20cm).

I am considering making a raised bed by 20cm but have no idea what
materials are required. I am on a tight budget since this is a
communal area. I would appreciate knowing how most folk would go about
making a corner raised bed, along with some guide prices for the
materials (I'll convert from $ if required). I will have to do the
work myself, so nothing permanent involving cement or other tricky
stuff please. Assume no power tools and the DIY skills of a total
amateur.

Either way, I have to have some edgeing if I can't have a raised bed,
as the grass comes right up to the plot and gets mowed.

The three new roses will be Floribunda, 90cm peak growth.