I use 500-ton Pinnace that deploys at the jump point. 10 years of deployment time and some amenities for the crew.

Pericles class Shuttle      500 tons       9 Crew       43.7 BP       TCS 10    TH 16    EM 0
1603 km/s      Armour 1-5       Shields 0-0       HTK 6      Sensors 5/0/0/0      DCR 1-20      PPV 0
Maint Life 39.31 Years     MSP 54    AFR 2%    IFR 0.0%    1YR 0    5YR 1    Max Repair 10 MSP
Lieutenant Commander    Control Rating 1   
Intended Deployment Time: 120 months    Morale Check Required   
● Medium Hydroponics-50    ● Recreation Room-25    ● Fitness Room-10    ● Escape Pod-5   

Honeywell Aero  Nuclear Thermal M16 (1)    Power 16    Fuel Use 180.0%    Signature 16    Explosion 10%
Fuel Capacity 104 000 Litres    Range 20.8 billion km (150 days at full power)

Silas-Nein Small Thermal Sensor 1-5 (1)     Sensitivity 5     Detect Sig Strength 1000:  17.7m km

This design is classed as a Fighter for production, combat and planetary interaction
This design is classed as a None for auto-assignment purposes

I believe the buoys are a good option, but I might quibble with easier to distribute: they've got a lot less tonnage but they can only be launched by a military ship, while commercial picket stations can be delivered by a commercial carrier (or tug). YMMV but I always appreciate the ability to solve my problems with a commercial hull.

Though if you're running missile warships through the area already, sticking a few buoys in their magazines isn't asking much. Or you could strap buoy launchers onto things like survey ships that are already obligated to be military.

Commercial station types like that really are useful for the fire-and-forget capability, but you have to accept that their sensors will not be very good. They'll let you know when somebody is visiting the picketed POI, but they're not going to offer much area intel.

Mine rate under 250 tons, so I can pack a bunch into one of my (also commercial) auxiliary carriers when I want to distribute them.

I have a fairly decent suite of sensors on my cruisers so i know they work

Im trying to build some small sensor stations near jump points to keep a beady eye on any neighbours - i create the station - add the sensors, but when i press 'no armour' so they can be space stations built with industry the sensors are wiped off the build and i cant add them back on until i untick the no armour box - can they not be unarmoured or are my sensors too heavy

"No armor" means a commercial space station, which can be built with planetary industry - this means you are limited to commercial components only, along with no engines.

Sensors larger than 1 HS are considered military components. Commercial ships must mount sensors of 1 HS or smaller size.