Keep in mind that the panel covering the hatch "garage" is fixed with some short screws only - not sure if the fixings are strong enough to attach a cannister liferaft.

I will probably fit some lashing points to the coaming around the coverpanel, to strap my valise-raft down when at sea. We are usually on the Dutch inland waters, so I don't bother bringing the liferaft along.

There's no 'best place' to site a raft, only the 'least worst'.  Excess weight at the extreme ends can cause awful hobby-horsing in a seaway and will strain the pushpit over time unless you're careful - but its by far the best location if you need to launch it quickly.  Towards midships is best for balancing the boat but only if you're sure you can launch it ok as it's not such an easy task as it might seem.  Attach it to the garage panel and it'll become self-launching sooner or later.  Cockpit lockers are a reasonable compromise - but only if all crew members can lift and handle it.  My 4-person valise now lives in a dedicated section of the shallow port-side cockpit locker together with the grab-bag but others will have different solutions that suit them best.