Short lens with 72mm filter

I'm after some lens advice. I need the shortest lens I can find, with a filter size no more than 72mm, in a Pentax mount. The restriction is because I'll be using it underwater, and that's the restriction of my housing. The structure of the housing also means my 10-17mm fisheye won't work, because it's not domed, and needs to screw an adaptor into the front of the lens.
I've been using the 18-55mm kit lens (see the shark in my portfolio), but something shorter and/or better quality would be ideal.
Probably not too fussed about manual or automatic focus (in case there's an older lens that fits the bill).
All the short lenses I can find have a filter size of 77mm. It's proving to be a very tricky google search....

Any of the pancakes?
40mm, 21mm, 70mm.

40mm £360 ish manuals on ebay cheaper.

Afer looking a bit harder, I've found a useful page for filter sizes link.
Seems like my only option shorter than I have now is a 16-45mm - is this going to be noticeably better quality than my 18-55mm kit?

When you say short, is this wide angle, rather than lens length protruding from the camera?

Just watch the 16-45 as it extends some way out, at the wide end. It is better than the 18-55 but not by a huge margin. The 18-55mm II or WR are both nipping at the 16-45mm heels.

Tamron 17-50mm is 67mm filter and has F2.8 which might help a little with limited light. The 16-45mm is F4.

I think it is noticeably better, but it's not night and day. It's sharper wide open, and there's significantly less vignetting and distortion than the kit.

The extra 2mm also gives a nice perspective. It's a quality lens.

If somebody has one to hand it would be interesting to measure it at 45mm and at 16mm just to see the length it extends.

Answer
at 16mm = 12.5cm
at 45mm = 9.3 cm

Yep, should have said I meant short focal length rather than physical length. Maximum length in the housing is 170mm. Might be a tight fit with 125mm of lens fitted to the camera.
Does the lens focus internally? Water pressure can put undue strain on the AF system if the front element is moving backwards and forwards.

The 15mm DA Ltd comes to mind, it accepts 49mm filters. However the retractable lens hood may not retract far enough.

Good point, had a look and it might work - I'll need to try it in person to see if the housing mount can screw onto the lens.
I think I might need to do a little saving, or ebay trawling, first - I was a bit shocked by the £600 price tag...

One thing to bear in mind is that the 16-45's main weakness is very noticeable purple fringing. I don't know if PF is as likely taking photos underwater as it is when taking photos OF water (water droplets being the traditional test subject), but if it is then probably not the lens to go for!

The Sigma 18-50 takes a 72mm filter, and also offers a constant f2.8 if required.

Adrian