I'm in the process of making one using flat channel ducting available from electrical dealers and places like B & Q. It's white plastic, can't remember the measurements offhand but it's about 2.5" deep, and 5" wide/long. Only extra work is to fill in each end or top and bottom, depending on orientation, but I used a small piece of teak routered around to fit.
It's much cheaper than a custom built intrument pod. I got the idea from an article in PBO some time ago. The only problem I have is that I'm fitting a Forward Looking Echosounder, which is, of course, too large for the standard ducting, so had to get a length on order (cost about £10, so still pretty cheap).
If you do need some of the larger sort, depending on the instruments involved, then pm me and, for the cost of the postage I'll send you the metre or so that I've got left over!