the frame you suggest will not be very strong, but if you added at the back something like this.
you would increase the strength, what would be ideal, is to have a full square frame so a piece of wood on all four sides including the base, if you used larger versions of the brackets above this would prove to be quite sturdy.
Wickes diy store are pretty good for timber, i recently completed a job that went in quite a prestigious music venue, i used wickes stud work timber 38mm x 63mm (thats the dimensions) a 2.4 metre length cost about two pounds fifty, these would be perfect and can be reused for other jobs.
when screwing it helps to pilot the hole, this basically means drilling out the hole in the first piece of timber that the screws will pass through, so if the body of a screw is 5mm wide drill a hole 4.5mm. this will make it easier to get a square and flush edge.
hope you get to grips with this, these are the basics of set building so once you understand this, you are off and away
adidat