how about making the item yourself out of grp, and it would be exactly what you are looking for as it would fit exactly, making a mould would be the most difficult thing to make, if you dont have the skills, look for a grp firm who could make one for you, you might have a bit of leg work to do to find someone, but sure that the finished item would be as good as you could get , compared with an off the shelf item....
you could make your own ... alternative to colvic's grp is buy a teak block slightly thicker than instrument depth reqd. plus enough to feed cables etc. you will need to drill out somewhere near 90mm holes for each instrument which can be done by hand using hole cutter but drill with care ... cut a backplate from ply or thin 5mm teak to cover all which can be easily sealed ... end product can look better than grp ... you need to find a local timber/ship wright with an odds & end bin wherein lurks a suitable, cheap, block of teak ......
White pvc rectangular section drain pipe (rain water down pipe) - easy to mount and cables inside. Cheap and as you usually have to buy 3m lenghts you have at least one spare and maybe two