I would get myself down to the local wood emporium and find some suitable size pine slats.
If you have a table saw even better.
You don't actually need an L shaped moulding. You need two bits - the side (the part that shows) and
the shelf (that the canvas rests on). Hammer and nails.
If you want all the frames the same depth on the outside, you would need the sides to be about 65-70 mm deep.
Make the shelf out of suitable size PSE timber and nail the sides to it. You have the option of fixing the shelf at a hight
to suit the thickness of the canvas. Fill the nail holes. They will blend into the texture of the wood.
Go over the sides with a wire wheel on a power drill to rough them up a bit. Easier to do this before you chop the frames.
Paint with ebony shellac and when that is dry, a coat of black paint. Quick buff up with dark wax and you are done.
A bit more work and a bit more messy than chopping finished moulding, but not as much faff as it may seem.
* If you don't want it quite as rustic as pine, Rose&Hollis do some handy square Obeche sections. There is one 9x73mm (F7) which would
serve for the sides. Another 12x47(F8) would make the shelf. You can paint straight on Obeche and it will look grainy. Wire brushing
will open the grain a bit, but don't lean on it as much as pine.