There are drawings of the rocket in John Lambert and Al Ross' Allied Coastal Forces of World War II Volume I. The rockets carried a parachute in their forward section.
Vosper boats either carried the ready rounds on racks on both sides of the wheelhouse (often with canvas covers), or in boxes in the same location (e.g MTB 224), or further forward in vertical boxes (e.g MTB 204). MTB 358 had boxes along the deck inboard of the 21-inch tubes, and the Type II Vospers had armoured boxes against the deckhouse.
British Power Boat Company 71ft 6in MGBs/MTBs carried them in similar sorts of racks or boxes, usually against the wheelhouse, with the launcher mounted on the starboard side of the power-operated 6pdr. Some BPBCo boats carried two launchers side-by-side.
Fairmile Ds were often fitted with several vertical racks forward of the deckhouse, or similar boxes to the Vosper and BPBCo boats, with either one or two launchers on their forward 6pdrs. A few Fairmile Bs carried launchers, with ready racks on the sides of the wheelhouse.
You'll probably find them on other boats - knowing the size of the rocket, look for similarly sized boxes near the launcher.