It is essential that the longest length is 1200mm if you require it to fit into a golf carry bag. This will include any of the protrusions that will be used as joins.
Now there are four ways to do this:
You can use round aluminium tubing and round the ends of your uprights to fit.
You can use square aluminium tubing
You can use two lengths of aluminium angles (as shown in the "Frame File")
You can use pins made from bolts or metal rod.

The thing I have found is that the length that fits over the other piece does not have to be all that long. 30 or 40mm ought to be sufficient. When I first made a booth I made it 120mm but found it difficult to pull apart so I cut it down and found that it was equally effective. Once the lazy tongs are extended and the cover of the booth is on everything gets held in place, the structural forces are all downward.
It must be an easy fit and not tight. So if you use one of the tubing methods sand the piece that fits in so that it slips in and out quite easily. If you use the metal pin method you want the hole to be slightly larger than the rod or bolt you use. I use a 6mm bolt glued into a 6mm hole, the other hole is 6.5mm(1/4 inch) so I have a 0.5mm clearance. (This applies to any bolts that go through the timber.) Round the end of the pin so there is no sharpness.
Now that I think about it there is no reason why the upper section should not slip over or into the lower. So this could be a decision that you could make.
The other important thing is to chamfer the ends of the uprights to avoid splitting. If you want to use the booth indoors you could put felt or rubber on the bottom. Sand all corners so that it is nice to handle. I oil my timber with linseed oil. You could even wax it I suppose. You want the setting up of your booth to be a pleasant tactile experience otherwise you will get into a bad mood and this will effect your show (just joking).