I have made my own,in every guise there is,even had a lad at a local council depot,who could get me the suitably shaped decal,made in traffolyte and marked up. You can then pot them up with a 2 part synthetic rubber,similar to what the broadband cable lads use.
Did a similar thing with a 7 pin trailer/vehicle tester,when i was a kid...the ones on ebay are not a patch on mine
...seem to remember getting a rollickin off me dad,as i used all his araldite up,potting it
I suppose it boils down to having something you can pass to others,and let them use,which means CE,and other approvals,which the advertised items should possess.
I once had a pal,who managed a yard full of trailer cabins,toilets,"welfare units",etc,and he asked me to cook up a way of speeding up,the apprentices weekly testing,of the lighting boards.
I devoted a day to it,and adapted a Wolf rechargeable lamp,which had a decent 12v battery,to which i added switching,some led's,and a 5m coil cord. The lad could do 50 lighting board checks in an afternoon,instead of 10,as no vehicle access was needed,and checks could be made on a suspect unit,whilst on a customers vehicle.
I told him to pay me what he thought it was worth,having used it a week
He rung me up and said he was staggered,just how much hassle it saved,and told me to come in,and pick up me "gift". On arriving,he presented me with a Mosa welder/generator,worth at the time,about fifteen hundred sovs,and said it was worth it
Long story/short - never underestimate,the value and worth,of a homemade device
