The most straightforward & "Hobie recommended" way is to mount it on a Hobie accessory mount. Glue down the mount, install a Scotty base, attach whatever fishfinder/electronics mount you want to the Scotty base. Ram & Scotty make several kinds of mounts. You can also mount to the forward hatch or bulkhead if you have good eyes & don't mind reaching a lot to fiddle with the unit, but I wouldn't recommend that.
I made a sideboard out of a small polyetheylene cutting board & attached it with straps to the various padeyes. Worked pretty well & was completely removable. You can also mount a swing arm for the transducer to it as well. I'd post a photo but I can't figure out how to from an iPhone. Google Mad Frog kayak accessories for basic idea.
You can put the battery in a dry bag or dry box and throw it in the front hatch, in you crate, hang it behind the mount, etc. I simply use a (10) AA cell holder with rechargeable batteries. The batteries, fuse, & excess wire all go in the dry bag. Cheap and easy. If I need fresh batteries (or forget to charge) I can grab some AA batteries from the store. Charged batteries last at least 10 hours with my simple Lowrance unit.