Open frame construction is not that difficult, if you can't square, invest in a mitre box, there's usually 3 slots 1 square and 2 at 45°. You clamp put your piece of wood in the U and either hold it or clamp it, the saw goes down the selected slot. Drill the side members to take wood screws and countersink. Hold or clamp the side and cross member and screw them together, repeat as necessary. A couple of diagonals may be needed to stop the structure twisting, if you don't get an accurate cut on them a good dollop of PVA and a couple of pins will sort it. Don't put the diagonals where they will foul the runners.