Since your flowmeter does not have a hose barb,you must buy it separately.
Make sure you buy female stainless steel or brass 1/4 G thread(BSPP) straight hose barb for your flowmeter Outlet.
Wide range of Stainless Steel Straight Female Hose Barbs. 15.000 products in stock and same day shipment
tameson.com
Don't buy NPT hose barb because male and female NPT hose barbs are for American regulators/flowmeters(CGA regulators/flowmeters), Your regulator/flowmeter is European(DIN standard),therefore,the outlet of your regulator has a G(BSPP) thread.
Here is a definitive guide to differentiate between BSPP (or GAS), BSPT, and NPT threads to help you get the best thread selection.
redfluid.es
A hose barb has two sides,the barbed side and the thread(male or female thread) side(the thread side has different standards such as NPT for US and Canada,BSPP(also called G thread)and BSPT for Europe.
The size of the barbed side depends on the ID size (inner diameter size)of the hose you want to use( for example Vizzy used 1/4 hose(6.3 mm ID size and 13.5mm OD size)) and the size of the thread side depends on the OD size(outside diameter size)of your regulator/flowmeter male outlet thread(your flowmeter is G 1/4 as you said)
Also 2 hose barbs(1 male and 1 female)+teflon tape+ 2 hose clamps or 2 air hose ferrules are used to connect two hoses together(Like Vizzy did).
NITROGEN GAS WITH EEBD HOOD (INERT GAS METHOD) before I start this guide, let’s be honest this method is expensive (approximately 600 to 800 dollars)and you need minimum 1 -2 months to source the equipments and nitrogen gas cylinder. My tip for you to do it cheap, purchase the Chinese products...
sanctioned-suicide.net
The flowmeter that GasMonkey suggested has a hose barb,so you don't need to worry about buying the right size hose barb but you should worry about buying a hose with the right ID size for the hose barb of that flowmeter