Nightsky, goto >
www1.jaycar.com.au
Cat No.,> GH1379. 12 volts out @ 5 AMPS for $29.95.
You need a plug pack unit of at least 1.5 or more AMPS output @ your required 12? volts.
The unit, (300mA), you posted will not supply enough AMPS to run your t'scope.
HTH.
L.
ps. Check your plug/socket requirements on the t'scope. Usually 2.5 mm "DC power" plug/socket.