I had one made to measure by a local joiner (there is a local college with a joinery section, so there are quite a few around). I have an idea he charged about £80. That's a ledged, braced and framed door, very popular for outbuildings.
I supplied the lock and hinges.
Try to get a local recommendation from someone you know and trust. Asvertising websites are not personal recommendations, even if they disguise themselves as such.
Two people can easily lift it on and off a car roof rack, one person, if stronger than average.
I've seen joinery workshops advertising on ebay at similar prices. Try to get one with mortice and tenon joints.
The diagonal braces are intended to prevent them sagging, but they do tend to, especially if exposed to the weather. If you use hook and band hinges it is easy to put extra stainless steel washers on the pins to lift them a bit.
It will last longer if protected from rain, even by a small canopy.
The timber will expand and contract with humidity, so rather than paint, which will crack, a wood preservative and/or breathing stain will look better. You can lift it off and lie it flat to treat.
I can suggest suitable locks and hinges if you want.