The vulnerable parts are the corners of the frames. Eric used a zigzag timber connector which was not clever on such small section wood as it tends to weaken the batten. A better method is a triangular metal mending plate screwed over the corner. You could use a triangular plywood peice but this wood need to be recessed to allow folding. This is the more elegant method.
Eric covered the booths in industrial nylon. This is fine, but is not lightproof. If used you need to tack blackout material on the inside of the bottom frame.
The triangular openings at either side of the booth look peculiar at first, but in practice prove very effective for puppets popping out etc. Actually I didn't like the look of the proscenium sides - two narrow. To make them wider reduces the playing space too much. I stripped them off altogether - just leaving slender upright pillars supporting the header board. This looked more aesthically pleasing and also made for much wider sight-lines.
Another way is to make an extra clip-on proscenium which adds width - but outwards, thus not reducing stage width. This way you can have a much better proportioned proscenium, but does mean that you have an extra piece to carry.
On one booth I made I used slightly wider square timber to make the frames - only by about a quarter inch. This made for a very stable structure but added a surprising amount to the weight.
Eric uses ball catches to lock the top frame to the lower. This is not very effective. A better method is to use a long strip of stick on velcro to upper and lower battens. This works a treat and locks the frame beautifully. For the velcro to stick the wood surface should be planed. Then give a thin coat of paint or varnish to the area to be velcroed. Stick down the velro - then protect the two ends by using a staple gun.
The effort you put into making this frame will be well worth it - it is a very clever design and will serve you well for years and years.