I think the biggest trick here is to know what is readily available off the shelf. If you measure the height of your panel and it is 530mm high or smaller then you can easily grab one off the shelf. Anything over 530mm tall for a p shaped bath and you are going to find it impossible to source one, unless of course you can track down the original supplier.
I paid £57 to buy a 530mm one off the shelf which if it was 50mm taller would have been a perfect fit but I couldn't find a nice way to adapt that one for my needs without destroying it in the process. If your looking for something less rustic you could build the frame from wood and then get some thin marine ply and bend it around the frame. That's the best way I can think of getting a smooth finish to the panel. Overall though I would recommend giving it a go if you have a load of reclaimed wood lying about and a jigsaw. It really isn't that difficult a job
I paid £57 to buy a 530mm one off the shelf which if it was 50mm taller would have been a perfect fit but I couldn't find a nice way to adapt that one for my needs without destroying it in the process. If your looking for something less rustic you could build the frame from wood and then get some thin marine ply and bend it around the frame. That's the best way I can think of getting a smooth finish to the panel. Overall though I would recommend giving it a go if you have a load of reclaimed wood lying about and a jigsaw. It really isn't that difficult a job