Sorry about repeated posts - but this is one closse to my heart: very nevrous about being away this holiday season too, but family committments, etc.
PIR's that are described as 'CMOS' will be 12v - although most technically work on anything from about 7v - to 16v, so are often setup with 9v batteries, like smoke alarms. some cheaper / older electronics is TTL (which uses 5v, or close) these often use 3x AA batteries - eg. radio-control door-bells.
I've considered large-scale lighting as described earlier, but we're pretty remote and get more wildlife (sheep, birds & trees moving in the wind) so if you're not careful all you'd get would be flat batteries over a week away.
If you're in a marina, or public busy towpath with many neighbours - lights and minor noise would be vey effective: Leicestershire Police were selling (£5 token donation) 'garden shed alarms' which work on a pull-cord. Very noisy, won't drain the battery, but you do have to set-up a cats-cradle of strings that you inevitable end-up setting the alarm off yourself.
House burgular alarms often have internal sirens that are really loud - discovered after setting one off once during a power cut.
They're designed to drive the intruder out by sheer discomfort ! (if the outside siren didn't un-nerve them).
I don't believe there's any law against this, although I stand to be advised if anyone knows different.
I'd suggest this would only work with an internal PIR or sensor though, as neighbours would be really upset with false alarms for wild-life at this level of noise. Potentially you could hide an activation switch in cratch or water locker, (or somewhere underdeck for Cruiser-style), or even conveniently inside an air-vent; its only gotta be a switch.
As well as Screwfix (they're stock changes too regularly to list) there's also Maplin, or RS Components - all with online shops.
Good luck with the improvements. (one and all as I think most of us have a similar problem).